diff --git a/go/packages/packagestest/expect.go b/go/packages/packagestest/expect.go
index 14e1fc01..0902057b 100644
--- a/go/packages/packagestest/expect.go
+++ b/go/packages/packagestest/expect.go
@@ -146,7 +146,11 @@ func (e *Exported) getNotes() error {
 	}
 	for _, pkg := range pkgs {
 		for _, filename := range pkg.GoFiles {
-			l, err := expect.Parse(e.fset, filename, nil)
+			content, err := e.FileContents(filename)
+			if err != nil {
+				return err
+			}
+			l, err := expect.Parse(e.fset, filename, content)
 			if err != nil {
 				return fmt.Errorf("Failed to extract expectations: %v", err)
 			}
@@ -337,7 +341,7 @@ func (e *Exported) rangeConverter(n *expect.Note, args []interface{}) (Range, []
 			return mark, args, nil
 		}
 	case string:
-		start, end, err := expect.MatchBefore(e.fset, e.fileContents, n.Pos, arg)
+		start, end, err := expect.MatchBefore(e.fset, e.FileContents, n.Pos, arg)
 		if err != nil {
 			return Range{}, nil, err
 		}
@@ -346,7 +350,7 @@ func (e *Exported) rangeConverter(n *expect.Note, args []interface{}) (Range, []
 		}
 		return Range{Start: start, End: end}, args, nil
 	case *regexp.Regexp:
-		start, end, err := expect.MatchBefore(e.fset, e.fileContents, n.Pos, arg)
+		start, end, err := expect.MatchBefore(e.fset, e.FileContents, n.Pos, arg)
 		if err != nil {
 			return Range{}, nil, err
 		}
diff --git a/go/packages/packagestest/export.go b/go/packages/packagestest/export.go
index 7214eddc..14a0ac15 100644
--- a/go/packages/packagestest/export.go
+++ b/go/packages/packagestest/export.go
@@ -39,6 +39,11 @@ type Module struct {
 	// be a string or byte slice, in which case it is the contents of the
 	// file, otherwise it must be a Writer function.
 	Files map[string]interface{}
+
+	// Overlay is the set of source file overlays for the module.
+	// The keys are the file fragment as in the Files configuration.
+	// The values are the in memory overlay content for the file.
+	Overlay map[string][]byte
 }
 
 // A Writer is a function that writes out a test file.
@@ -53,13 +58,15 @@ type Exported struct {
 	// Exactly what it will contain varies depending on the Exporter being used.
 	Config *packages.Config
 
-	temp     string                       // the temporary directory that was exported to
-	primary  string                       // the first non GOROOT module that was exported
-	written  map[string]map[string]string // the full set of exported files
-	fset     *token.FileSet               // The file set used when parsing expectations
-	notes    []*expect.Note               // The list of expectations extracted from go source files
-	markers  map[string]Range             // The set of markers extracted from go source files
-	contents map[string][]byte
+	// Modules is the module description that was used to produce this exported data set.
+	Modules []Module
+
+	temp    string                       // the temporary directory that was exported to
+	primary string                       // the first non GOROOT module that was exported
+	written map[string]map[string]string // the full set of exported files
+	fset    *token.FileSet               // The file set used when parsing expectations
+	notes   []*expect.Note               // The list of expectations extracted from go source files
+	markers map[string]Range             // The set of markers extracted from go source files
 }
 
 // Exporter implementations are responsible for converting from the generic description of some
@@ -126,14 +133,15 @@ func Export(t testing.TB, exporter Exporter, modules []Module) *Exported {
 	}
 	exported := &Exported{
 		Config: &packages.Config{
-			Dir: temp,
-			Env: append(os.Environ(), "GOPACKAGESDRIVER=off"),
+			Dir:     temp,
+			Env:     append(os.Environ(), "GOPACKAGESDRIVER=off"),
+			Overlay: make(map[string][]byte),
 		},
-		temp:     temp,
-		primary:  modules[0].Name,
-		written:  map[string]map[string]string{},
-		fset:     token.NewFileSet(),
-		contents: map[string][]byte{},
+		Modules: modules,
+		temp:    temp,
+		primary: modules[0].Name,
+		written: map[string]map[string]string{},
+		fset:    token.NewFileSet(),
 	}
 	defer func() {
 		if t.Failed() || t.Skipped() {
@@ -165,6 +173,10 @@ func Export(t testing.TB, exporter Exporter, modules []Module) *Exported {
 				t.Fatalf("Invalid type %T in files, must be string or Writer", value)
 			}
 		}
+		for fragment, value := range module.Overlay {
+			fullpath := exporter.Filename(exported, module.Name, filepath.FromSlash(fragment))
+			exported.Config.Overlay[fullpath] = value
+		}
 	}
 	if err := exporter.Finalize(exported); err != nil {
 		t.Fatal(err)
@@ -282,8 +294,11 @@ func (e *Exported) File(module, fragment string) string {
 	return ""
 }
 
-func (e *Exported) fileContents(filename string) ([]byte, error) {
-	if content, found := e.contents[filename]; found {
+// FileContents returns the contents of the specified file.
+// It will use the overlay if the file is present, otherwise it will read it
+// from disk.
+func (e *Exported) FileContents(filename string) ([]byte, error) {
+	if content, found := e.Config.Overlay[filename]; found {
 		return content, nil
 	}
 	content, err := ioutil.ReadFile(filename)
diff --git a/go/packages/packagestest/export_test.go b/go/packages/packagestest/export_test.go
index decfd7c0..e2a2355f 100644
--- a/go/packages/packagestest/export_test.go
+++ b/go/packages/packagestest/export_test.go
@@ -5,7 +5,6 @@
 package packagestest_test
 
 import (
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"testing"
@@ -17,7 +16,11 @@ var testdata = []packagestest.Module{{
 	Name: "golang.org/fake1",
 	Files: map[string]interface{}{
 		"a.go": packagestest.Symlink("testdata/a.go"),
-		"b.go": "package fake1",
+		"b.go": "invalid file contents",
+	},
+	Overlay: map[string][]byte{
+		"b.go": []byte("package fake1"),
+		"c.go": []byte("package fake1"),
 	},
 }, {
 	Name: "golang.org/fake2",
@@ -33,7 +36,7 @@ var testdata = []packagestest.Module{{
 
 type fileTest struct {
 	module, fragment, expect string
-	check                    func(t *testing.T, filename string)
+	check                    func(t *testing.T, exported *packagestest.Exported, filename string)
 }
 
 func checkFiles(t *testing.T, exported *packagestest.Exported, tests []fileTest) {
@@ -46,14 +49,14 @@ func checkFiles(t *testing.T, exported *packagestest.Exported, tests []fileTest)
 			t.Errorf("Got file %v, expected %v", got, expect)
 		}
 		if test.check != nil {
-			test.check(t, got)
+			test.check(t, exported, got)
 		}
 	}
 }
 
-func checkLink(expect string) func(t *testing.T, filename string) {
+func checkLink(expect string) func(t *testing.T, exported *packagestest.Exported, filename string) {
 	expect = filepath.FromSlash(expect)
-	return func(t *testing.T, filename string) {
+	return func(t *testing.T, exported *packagestest.Exported, filename string) {
 		if target, err := os.Readlink(filename); err != nil {
 			t.Errorf("Error checking link %v: %v", filename, err)
 		} else if target != expect {
@@ -62,9 +65,9 @@ func checkLink(expect string) func(t *testing.T, filename string) {
 	}
 }
 
-func checkContent(expect string) func(t *testing.T, filename string) {
-	return func(t *testing.T, filename string) {
-		if content, err := ioutil.ReadFile(filename); err != nil {
+func checkContent(expect string) func(t *testing.T, exported *packagestest.Exported, filename string) {
+	return func(t *testing.T, exported *packagestest.Exported, filename string) {
+		if content, err := exported.FileContents(filename); err != nil {
 			t.Errorf("Error reading %v: %v", filename, err)
 		} else if string(content) != expect {
 			t.Errorf("Content of %v does not match, got %v expected %v", filename, string(content), expect)