go/gcexportdata: support old export data (again), needed by AE1.6

+ test

Change-Id: I0842795ece634e4441b17a10009d26bfc2a8481a
Reviewed-on: https://go-review.googlesource.com/31818
Reviewed-by: Robert Griesemer <gri@golang.org>

go/gcexportdata/gcexportdata.go

@@ -25,6 +25,7 @@ package gcexportdata // import "golang.org/x/tools/go/gcexportdata"
 
 import (
 	"bufio"
+	"bytes"
 	"fmt"
 	"go/token"
 	"go/types"
@@ -76,6 +77,13 @@ func Read(in io.Reader, fset *token.FileSet, imports map[string]*types.Package,
 	if err != nil {
 		return nil, fmt.Errorf("reading export data for %q: %v", path, err)
 	}
+
+	// The App Engine Go runtime v1.6 uses the old export data format.
+	// TODO(adonovan): delete once v1.7 has been around for a while.
+	if bytes.HasPrefix(data, []byte("package ")) {
+		return gcimporter.ImportData(imports, path, path, bytes.NewReader(data))
+	}
+
 	_, pkg, err := gcimporter.BImportData(fset, imports, data, path)
 	return pkg, err
 }

go/gcexportdata/gcexportdata_test.go

@@ -0,0 +1,41 @@
+package gcexportdata_test
+
+import (
+	"go/token"
+	"go/types"
+	"log"
+	"os"
+	"testing"
+
+	"golang.org/x/tools/go/gcexportdata"
+)
+
+// Test to ensure that gcexportdata can read files produced by App
+// Engine Go runtime v1.6.
+func TestAppEngine16(t *testing.T) {
+	// Open and read the file.
+	f, err := os.Open("testdata/errors-ae16.a")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	r, err := gcexportdata.NewReader(f)
+	if err != nil {
+		log.Fatalf("reading export data: %v", err)
+	}
+
+	// Decode the export data.
+	fset := token.NewFileSet()
+	imports := make(map[string]*types.Package)
+	pkg, err := gcexportdata.Read(r, fset, imports, "errors")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Print package information.
+	got := pkg.Scope().Lookup("New").Type().String()
+	want := "func(text string) error"
+	if got != want {
+		t.Errorf("New.Type = %s, want %s", got, want)
+	}
+}