diff --git a/go/gcexportdata/gcexportdata.go b/go/gcexportdata/gcexportdata.go
index f8363d8..0e42514 100644
--- a/go/gcexportdata/gcexportdata.go
+++ b/go/gcexportdata/gcexportdata.go
@@ -100,10 +100,8 @@
 // Write writes encoded type information for the specified package to out.
 // The FileSet provides file position information for named objects.
 func Write(out io.Writer, fset *token.FileSet, pkg *types.Package) error {
-	b, err := gcimporter.IExportData(fset, pkg)
-	if err != nil {
+	if _, err := io.WriteString(out, "i"); err != nil {
 		return err
 	}
-	_, err = out.Write(b)
-	return err
+	return gcimporter.IExportData(out, fset, pkg)
 }
diff --git a/go/internal/gcimporter/iexport.go b/go/internal/gcimporter/iexport.go
index 4be32a2..144948c 100644
--- a/go/internal/gcimporter/iexport.go
+++ b/go/internal/gcimporter/iexport.go
@@ -25,12 +25,12 @@
 // 0: Go1.11 encoding
 const iexportVersion = 0
 
-// IExportData returns the binary export data for pkg.
+// IExportData writes indexed export data for pkg to out.
 //
 // If no file set is provided, position info will be missing.
 // The package path of the top-level package will not be recorded,
 // so that calls to IImportData can override with a provided package path.
-func IExportData(fset *token.FileSet, pkg *types.Package) (b []byte, err error) {
+func IExportData(out io.Writer, fset *token.FileSet, pkg *types.Package) (err error) {
 	defer func() {
 		if e := recover(); e != nil {
 			if ierr, ok := e.(internalError); ok {
@@ -43,7 +43,6 @@
 	}()
 
 	p := iexporter{
-		out:         bytes.NewBuffer(nil),
 		fset:        fset,
 		allPkgs:     map[*types.Package]bool{},
 		stringIndex: map[string]uint64{},
@@ -80,17 +79,16 @@
 
 	// Assemble header.
 	var hdr intWriter
-	hdr.WriteByte('i')
 	hdr.uint64(iexportVersion)
 	hdr.uint64(uint64(p.strings.Len()))
 	hdr.uint64(dataLen)
 
 	// Flush output.
-	io.Copy(p.out, &hdr)
-	io.Copy(p.out, &p.strings)
-	io.Copy(p.out, &p.data0)
+	io.Copy(out, &hdr)
+	io.Copy(out, &p.strings)
+	io.Copy(out, &p.data0)
 
-	return p.out.Bytes(), nil
+	return nil
 }
 
 // writeIndex writes out an object index. mainIndex indicates whether
diff --git a/go/internal/gcimporter/iexport_test.go b/go/internal/gcimporter/iexport_test.go
index 5024570..b85bf1f 100644
--- a/go/internal/gcimporter/iexport_test.go
+++ b/go/internal/gcimporter/iexport_test.go
@@ -9,6 +9,7 @@
 package gcimporter_test
 
 import (
+	"bytes"
 	"fmt"
 	"go/ast"
 	"go/build"
@@ -28,6 +29,14 @@
 	"golang.org/x/tools/go/loader"
 )
 
+func iexport(fset *token.FileSet, pkg *types.Package) ([]byte, error) {
+	var buf bytes.Buffer
+	if err := gcimporter.IExportData(&buf, fset, pkg); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
 func TestIExportData_stdlib(t *testing.T) {
 	if runtime.Compiler == "gccgo" {
 		t.Skip("gccgo standard library is inaccessible")
@@ -84,15 +93,10 @@
 		if info.Files == nil {
 			continue // empty directory
 		}
-		exportdata, err := gcimporter.IExportData(conf.Fset, pkg)
+		exportdata, err := iexport(conf.Fset, pkg)
 		if err != nil {
 			t.Fatal(err)
 		}
-		if exportdata[0] == 'i' {
-			exportdata = exportdata[1:] // trim the 'i' in the header
-		} else {
-			t.Fatalf("unexpected first character of export data: %v", exportdata[0])
-		}
 
 		imports := make(map[string]*types.Package)
 		fset2 := token.NewFileSet()
@@ -151,15 +155,10 @@
 	}
 
 	// export
-	exportdata, err := gcimporter.IExportData(fset1, pkg)
+	exportdata, err := iexport(fset1, pkg)
 	if err != nil {
 		t.Fatal(err)
 	}
-	if exportdata[0] == 'i' {
-		exportdata = exportdata[1:] // trim the 'i' in the header
-	} else {
-		t.Fatalf("unexpected first character of export data: %v", exportdata[0])
-	}
 
 	// import
 	imports := make(map[string]*types.Package)
@@ -199,15 +198,10 @@
 
 	// export
 	// use a nil fileset here to confirm that it doesn't panic
-	exportdata, err := gcimporter.IExportData(nil, pkg1)
+	exportdata, err := iexport(nil, pkg1)
 	if err != nil {
 		t.Fatal(err)
 	}
-	if exportdata[0] == 'i' {
-		exportdata = exportdata[1:] // trim the 'i' in the header
-	} else {
-		t.Fatalf("unexpected first character of export data: %v", exportdata[0])
-	}
 
 	// import
 	imports := make(map[string]*types.Package)