diff --git a/internal/wire/testdata/BuildTagsMainOnly/foo/wire.go b/internal/wire/testdata/BuildTagsMainOnly/foo/wire.go
index 70b68af..4f5bec6 100644
--- a/internal/wire/testdata/BuildTagsMainOnly/foo/wire.go
+++ b/internal/wire/testdata/BuildTagsMainOnly/foo/wire.go
@@ -17,8 +17,7 @@
 package main
 
 import (
-	"bar"
-
+	"example.com/bar"
 	"github.com/google/go-cloud/wire"
 )
 
diff --git a/internal/wire/testdata/BuildTagsMainOnly/pkg b/internal/wire/testdata/BuildTagsMainOnly/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/BuildTagsMainOnly/pkg
+++ b/internal/wire/testdata/BuildTagsMainOnly/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/BuildTagsRelativePkg/foo/wire.go b/internal/wire/testdata/BuildTagsRelativePkg/foo/wire.go
index 70b68af..4f5bec6 100644
--- a/internal/wire/testdata/BuildTagsRelativePkg/foo/wire.go
+++ b/internal/wire/testdata/BuildTagsRelativePkg/foo/wire.go
@@ -17,8 +17,7 @@
 package main
 
 import (
-	"bar"
-
+	"example.com/bar"
 	"github.com/google/go-cloud/wire"
 )
 
diff --git a/internal/wire/testdata/BuildTagsRelativePkg/pkg b/internal/wire/testdata/BuildTagsRelativePkg/pkg
index b10f96c..6b704cd 100644
--- a/internal/wire/testdata/BuildTagsRelativePkg/pkg
+++ b/internal/wire/testdata/BuildTagsRelativePkg/pkg
@@ -1 +1 @@
-./foo
+./example.com/foo
diff --git a/internal/wire/testdata/Chain/pkg b/internal/wire/testdata/Chain/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/Chain/pkg
+++ b/internal/wire/testdata/Chain/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/Cleanup/pkg b/internal/wire/testdata/Cleanup/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/Cleanup/pkg
+++ b/internal/wire/testdata/Cleanup/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/CopyOtherDecls/pkg b/internal/wire/testdata/CopyOtherDecls/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/CopyOtherDecls/pkg
+++ b/internal/wire/testdata/CopyOtherDecls/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/Cycle/pkg b/internal/wire/testdata/Cycle/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/Cycle/pkg
+++ b/internal/wire/testdata/Cycle/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/EmptyVar/pkg b/internal/wire/testdata/EmptyVar/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/EmptyVar/pkg
+++ b/internal/wire/testdata/EmptyVar/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/ExportedValue/foo/wire.go b/internal/wire/testdata/ExportedValue/foo/wire.go
index ea0bb83..e51f468 100644
--- a/internal/wire/testdata/ExportedValue/foo/wire.go
+++ b/internal/wire/testdata/ExportedValue/foo/wire.go
@@ -17,7 +17,7 @@
 package main
 
 import (
-	"bar"
+	"example.com/bar"
 	"github.com/google/go-cloud/wire"
 )
 
diff --git a/internal/wire/testdata/ExportedValue/pkg b/internal/wire/testdata/ExportedValue/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/ExportedValue/pkg
+++ b/internal/wire/testdata/ExportedValue/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/ExportedValue/want/wire_gen.go b/internal/wire/testdata/ExportedValue/want/wire_gen.go
index 241c560..a340ecb 100644
--- a/internal/wire/testdata/ExportedValue/want/wire_gen.go
+++ b/internal/wire/testdata/ExportedValue/want/wire_gen.go
@@ -6,7 +6,7 @@
 package main
 
 import (
-	bar "bar"
+	bar "example.com/bar"
 )
 
 // Injectors from wire.go:
diff --git a/internal/wire/testdata/ExportedValueDifferentPackage/foo/wire.go b/internal/wire/testdata/ExportedValueDifferentPackage/foo/wire.go
index a40390d..4f08716 100644
--- a/internal/wire/testdata/ExportedValueDifferentPackage/foo/wire.go
+++ b/internal/wire/testdata/ExportedValueDifferentPackage/foo/wire.go
@@ -19,7 +19,7 @@ package main
 import (
 	"os"
 
-	"bar"
+	"example.com/bar"
 	"github.com/google/go-cloud/wire"
 )
 
diff --git a/internal/wire/testdata/ExportedValueDifferentPackage/pkg b/internal/wire/testdata/ExportedValueDifferentPackage/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/ExportedValueDifferentPackage/pkg
+++ b/internal/wire/testdata/ExportedValueDifferentPackage/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/ImportedInterfaceBinding/bar/bar.go b/internal/wire/testdata/ImportedInterfaceBinding/bar/bar.go
index f7cd1cf..7771f90 100644
--- a/internal/wire/testdata/ImportedInterfaceBinding/bar/bar.go
+++ b/internal/wire/testdata/ImportedInterfaceBinding/bar/bar.go
@@ -17,7 +17,7 @@ package main
 import (
 	"fmt"
 
-	"foo"
+	"example.com/foo"
 	"github.com/google/go-cloud/wire"
 )
 
diff --git a/internal/wire/testdata/ImportedInterfaceBinding/bar/wire.go b/internal/wire/testdata/ImportedInterfaceBinding/bar/wire.go
index 870d2ee..b813b54 100644
--- a/internal/wire/testdata/ImportedInterfaceBinding/bar/wire.go
+++ b/internal/wire/testdata/ImportedInterfaceBinding/bar/wire.go
@@ -17,7 +17,7 @@
 package main
 
 import (
-	"foo"
+	"example.com/foo"
 	"github.com/google/go-cloud/wire"
 )
 
diff --git a/internal/wire/testdata/ImportedInterfaceBinding/pkg b/internal/wire/testdata/ImportedInterfaceBinding/pkg
index 5716ca5..923cd07 100644
--- a/internal/wire/testdata/ImportedInterfaceBinding/pkg
+++ b/internal/wire/testdata/ImportedInterfaceBinding/pkg
@@ -1 +1 @@
-bar
+example.com/bar
diff --git a/internal/wire/testdata/ImportedInterfaceBinding/want/wire_gen.go b/internal/wire/testdata/ImportedInterfaceBinding/want/wire_gen.go
index 1b369d9..02f1b22 100644
--- a/internal/wire/testdata/ImportedInterfaceBinding/want/wire_gen.go
+++ b/internal/wire/testdata/ImportedInterfaceBinding/want/wire_gen.go
@@ -6,7 +6,7 @@
 package main
 
 import (
-	foo "foo"
+	foo "example.com/foo"
 )
 
 // Injectors from wire.go:
diff --git a/internal/wire/testdata/InjectInput/pkg b/internal/wire/testdata/InjectInput/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/InjectInput/pkg
+++ b/internal/wire/testdata/InjectInput/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/InjectInputConflict/pkg b/internal/wire/testdata/InjectInputConflict/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/InjectInputConflict/pkg
+++ b/internal/wire/testdata/InjectInputConflict/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/InjectWithPanic/pkg b/internal/wire/testdata/InjectWithPanic/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/InjectWithPanic/pkg
+++ b/internal/wire/testdata/InjectWithPanic/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/InterfaceBinding/pkg b/internal/wire/testdata/InterfaceBinding/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/InterfaceBinding/pkg
+++ b/internal/wire/testdata/InterfaceBinding/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/InterfaceBindingReuse/pkg b/internal/wire/testdata/InterfaceBindingReuse/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/InterfaceBindingReuse/pkg
+++ b/internal/wire/testdata/InterfaceBindingReuse/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/InterfaceValue/pkg b/internal/wire/testdata/InterfaceValue/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/InterfaceValue/pkg
+++ b/internal/wire/testdata/InterfaceValue/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/InterfaceValueInvalidArg0/pkg b/internal/wire/testdata/InterfaceValueInvalidArg0/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/InterfaceValueInvalidArg0/pkg
+++ b/internal/wire/testdata/InterfaceValueInvalidArg0/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/InterfaceValueNotEnoughArgs/pkg b/internal/wire/testdata/InterfaceValueNotEnoughArgs/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/InterfaceValueNotEnoughArgs/pkg
+++ b/internal/wire/testdata/InterfaceValueNotEnoughArgs/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/MultipleMissingInputs/pkg b/internal/wire/testdata/MultipleMissingInputs/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/MultipleMissingInputs/pkg
+++ b/internal/wire/testdata/MultipleMissingInputs/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/MultipleMissingInputs/want/wire_errs.txt b/internal/wire/testdata/MultipleMissingInputs/want/wire_errs.txt
index 183f6f4..1d3dce6 100644
--- a/internal/wire/testdata/MultipleMissingInputs/want/wire_errs.txt
+++ b/internal/wire/testdata/MultipleMissingInputs/want/wire_errs.txt
@@ -1,2 +1,2 @@
-no provider found for foo.Foo
-no provider found for foo.Bar
+no provider found for example.com/foo.Foo
+no provider found for example.com/foo.Bar
diff --git a/internal/wire/testdata/NamingWorstCase/pkg b/internal/wire/testdata/NamingWorstCase/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/NamingWorstCase/pkg
+++ b/internal/wire/testdata/NamingWorstCase/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/NamingWorstCaseAllInOne/pkg b/internal/wire/testdata/NamingWorstCaseAllInOne/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/NamingWorstCaseAllInOne/pkg
+++ b/internal/wire/testdata/NamingWorstCaseAllInOne/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/NiladicIdentity/pkg b/internal/wire/testdata/NiladicIdentity/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/NiladicIdentity/pkg
+++ b/internal/wire/testdata/NiladicIdentity/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/NiladicValue/pkg b/internal/wire/testdata/NiladicValue/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/NiladicValue/pkg
+++ b/internal/wire/testdata/NiladicValue/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/NoImplicitInterface/pkg b/internal/wire/testdata/NoImplicitInterface/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/NoImplicitInterface/pkg
+++ b/internal/wire/testdata/NoImplicitInterface/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/NoInjectParamNames/pkg b/internal/wire/testdata/NoInjectParamNames/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/NoInjectParamNames/pkg
+++ b/internal/wire/testdata/NoInjectParamNames/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/NoopBuild/pkg b/internal/wire/testdata/NoopBuild/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/NoopBuild/pkg
+++ b/internal/wire/testdata/NoopBuild/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/PartialCleanup/pkg b/internal/wire/testdata/PartialCleanup/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/PartialCleanup/pkg
+++ b/internal/wire/testdata/PartialCleanup/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/PkgImport/foo/foo.go b/internal/wire/testdata/PkgImport/foo/foo.go
index 2d755a8..228c574 100644
--- a/internal/wire/testdata/PkgImport/foo/foo.go
+++ b/internal/wire/testdata/PkgImport/foo/foo.go
@@ -17,7 +17,7 @@ package main
 import (
 	"fmt"
 
-	"bar"
+	"example.com/bar"
 	"github.com/google/go-cloud/wire"
 )
 
diff --git a/internal/wire/testdata/PkgImport/pkg b/internal/wire/testdata/PkgImport/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/PkgImport/pkg
+++ b/internal/wire/testdata/PkgImport/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/PkgImport/want/wire_gen.go b/internal/wire/testdata/PkgImport/want/wire_gen.go
index 90c41c4..a4e44b0 100644
--- a/internal/wire/testdata/PkgImport/want/wire_gen.go
+++ b/internal/wire/testdata/PkgImport/want/wire_gen.go
@@ -6,7 +6,7 @@
 package main
 
 import (
-	bar "bar"
+	bar "example.com/bar"
 )
 
 // Injectors from wire.go:
diff --git a/internal/wire/testdata/ReturnError/pkg b/internal/wire/testdata/ReturnError/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/ReturnError/pkg
+++ b/internal/wire/testdata/ReturnError/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/Struct/pkg b/internal/wire/testdata/Struct/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/Struct/pkg
+++ b/internal/wire/testdata/Struct/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/StructPointer/pkg b/internal/wire/testdata/StructPointer/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/StructPointer/pkg
+++ b/internal/wire/testdata/StructPointer/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/TwoDeps/pkg b/internal/wire/testdata/TwoDeps/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/TwoDeps/pkg
+++ b/internal/wire/testdata/TwoDeps/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/UnexportedValue/foo/wire.go b/internal/wire/testdata/UnexportedValue/foo/wire.go
index ea0bb83..e51f468 100644
--- a/internal/wire/testdata/UnexportedValue/foo/wire.go
+++ b/internal/wire/testdata/UnexportedValue/foo/wire.go
@@ -17,7 +17,7 @@
 package main
 
 import (
-	"bar"
+	"example.com/bar"
 	"github.com/google/go-cloud/wire"
 )
 
diff --git a/internal/wire/testdata/UnexportedValue/pkg b/internal/wire/testdata/UnexportedValue/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/UnexportedValue/pkg
+++ b/internal/wire/testdata/UnexportedValue/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/UnusedProviders/pkg b/internal/wire/testdata/UnusedProviders/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/UnusedProviders/pkg
+++ b/internal/wire/testdata/UnusedProviders/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/UnusedProviders/want/wire_errs.txt b/internal/wire/testdata/UnusedProviders/want/wire_errs.txt
index 13d8c09..d096736 100644
--- a/internal/wire/testdata/UnusedProviders/want/wire_errs.txt
+++ b/internal/wire/testdata/UnusedProviders/want/wire_errs.txt
@@ -1,4 +1,4 @@
 unused provider set "unusedSet"
 unused provider "provideUnused"
 unused value of type string
-unused interface binding to type foo.Fooer
+unused interface binding to type example.com/foo.Fooer
diff --git a/internal/wire/testdata/ValueChain/pkg b/internal/wire/testdata/ValueChain/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/ValueChain/pkg
+++ b/internal/wire/testdata/ValueChain/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/ValueConversion/pkg b/internal/wire/testdata/ValueConversion/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/ValueConversion/pkg
+++ b/internal/wire/testdata/ValueConversion/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/ValueIsInterfaceValue/pkg b/internal/wire/testdata/ValueIsInterfaceValue/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/ValueIsInterfaceValue/pkg
+++ b/internal/wire/testdata/ValueIsInterfaceValue/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/VarValue/pkg b/internal/wire/testdata/VarValue/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/VarValue/pkg
+++ b/internal/wire/testdata/VarValue/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/Vendor/foo/vendor/bar/bar.go b/internal/wire/testdata/Vendor/foo/vendor/example.com/bar/bar.go
similarity index 100%
rename from internal/wire/testdata/Vendor/foo/vendor/bar/bar.go
rename to internal/wire/testdata/Vendor/foo/vendor/example.com/bar/bar.go
diff --git a/internal/wire/testdata/Vendor/foo/wire.go b/internal/wire/testdata/Vendor/foo/wire.go
index 68a1abb..b58e2f7 100644
--- a/internal/wire/testdata/Vendor/foo/wire.go
+++ b/internal/wire/testdata/Vendor/foo/wire.go
@@ -17,7 +17,7 @@
 package main
 
 import (
-	"bar"
+	"example.com/bar"
 	"github.com/google/go-cloud/wire"
 )
 
diff --git a/internal/wire/testdata/Vendor/pkg b/internal/wire/testdata/Vendor/pkg
index 257cc56..f7a5c8c 100644
--- a/internal/wire/testdata/Vendor/pkg
+++ b/internal/wire/testdata/Vendor/pkg
@@ -1 +1 @@
-foo
+example.com/foo
diff --git a/internal/wire/testdata/Vendor/want/wire_gen.go b/internal/wire/testdata/Vendor/want/wire_gen.go
index adc0279..605d593 100644
--- a/internal/wire/testdata/Vendor/want/wire_gen.go
+++ b/internal/wire/testdata/Vendor/want/wire_gen.go
@@ -6,7 +6,7 @@
 package main
 
 import (
-	bar "bar"
+	bar "example.com/bar"
 )
 
 // Injectors from wire.go:
diff --git a/internal/wire/wire_test.go b/internal/wire/wire_test.go
index 38a43e5..90ed012 100644
--- a/internal/wire/wire_test.go
+++ b/internal/wire/wire_test.go
@@ -71,6 +71,10 @@ func TestWire(t *testing.T) {
 	for _, test := range tests {
 		test := test
 		t.Run(test.name, func(t *testing.T) {
+			if test.name == "Vendor" && os.Getenv("GO111MODULE") != "off" {
+				// TODO: Remove the GO111MODULE check when it is not relevant (maybe after Go 1.12).
+				t.Skip("Skipped testing for vendored package for Go module turned on, see https://github.com/google/go-cloud/issues/326")
+			}
 			t.Parallel()
 
 			// Run Wire from a fake build context.
@@ -147,6 +151,9 @@ func goBuildCheck(test *testCase, wd string, bctx *build.Context, gen []byte) er
 			return err
 		}
 	}
+	if err := writeGoMod(gopath); err != nil {
+		return err
+	}
 	testExePath := filepath.Join(gopath, "bin", "testprog")
 	realBuildCtx := &build.Context{
 		GOARCH:      bctx.GOARCH,
@@ -158,7 +165,8 @@ func goBuildCheck(test *testCase, wd string, bctx *build.Context, gen []byte) er
 		BuildTags:   bctx.BuildTags,
 		ReleaseTags: bctx.ReleaseTags,
 	}
-	if err := runGo(realBuildCtx, "build", "-o", testExePath, genPkg.ImportPath); err != nil {
+	buildDir := filepath.Join(gopath, "src", genPkg.ImportPath)
+	if err := runGo(realBuildCtx, buildDir, "build", "-o", testExePath); err != nil {
 		return fmt.Errorf("build: %v", err)
 	}
 
@@ -342,7 +350,7 @@ func loadTestCase(root string, wireGoSrc []byte) (*testCase, error) {
 		if !*setup.Record {
 			wantWireOutput, err = ioutil.ReadFile(filepath.Join(root, "want", "wire_gen.go"))
 			if err != nil {
-				return nil, fmt.Errorf("load test case %s: %v. If this is a new testcase, run with -record to generate the wire_gen.go file.", name, err)
+				return nil, fmt.Errorf("load test case %s: %v, if this is a new testcase, run with -record to generate the wire_gen.go file", name, err)
 			}
 		}
 		wantProgramOutput, err = ioutil.ReadFile(filepath.Join(root, "want", "program_out.txt"))
@@ -368,7 +376,7 @@ func loadTestCase(root string, wireGoSrc []byte) (*testCase, error) {
 		if err != nil {
 			return err
 		}
-		goFiles[rel] = data
+		goFiles[filepath.Join("example.com", rel)] = data
 		return nil
 	})
 	if err != nil {
@@ -576,10 +584,45 @@ func (test *testCase) materialize(gopath string) error {
 	return nil
 }
 
-func runGo(bctx *build.Context, args ...string) error {
+// writeGoMod generates go.mod files for the test package and its dependency.
+// The file structure looks like:
+//
+//	gopath/src/
+//
+//		example.com/
+//
+//			go.mod
+//				replaces dependency with local copied one
+//
+//			... (Packages to be built and tested)
+//				any Go files copied recursively
+//
+//		github.com/google/go-cloud/
+//
+//			go.mod
+//
+//			... (Dependency files copied)
+func writeGoMod(gopath string) error {
+	importPath := "example.com"
+	depPath := "github.com/google/go-cloud"
+	depLoc := filepath.Join(gopath, "src", filepath.FromSlash(depPath))
+	example := fmt.Sprintf("module %s\n\nreplace %s => %s\n", importPath, depPath, depLoc)
+	gomod := filepath.Join(gopath, "src", importPath, "go.mod")
+	if err := ioutil.WriteFile(gomod, []byte(example), 0666); err != nil {
+		return fmt.Errorf("generate go.mod for %s: %v", gomod, err)
+	}
+	if err := ioutil.WriteFile(filepath.Join(depLoc, "go.mod"), []byte("module "+depPath), 0666); err != nil {
+		return fmt.Errorf("generate go.mod for %s: %v", depPath, err)
+	}
+	return nil
+}
+
+// runGo runs a go command in dir.
+func runGo(bctx *build.Context, dir string, args ...string) error {
 	exe := filepath.Join(bctx.GOROOT, "bin", "go")
 	c := exec.Command(exe, args...)
 	c.Env = append(os.Environ(), "GOROOT="+bctx.GOROOT, "GOARCH="+bctx.GOARCH, "GOOS="+bctx.GOOS, "GOPATH="+bctx.GOPATH)
+	c.Dir = dir
 	if bctx.CgoEnabled {
 		c.Env = append(c.Env, "CGO_ENABLED=1")
 	} else {