encoding/xml: flush buffer after encoding token

R=rsc, bradfitz, adg
CC=golang-dev
https://golang.org/cl/13004046

src/pkg/encoding/xml/marshal.go

@@ -196,7 +196,6 @@ func (enc *Encoder) EncodeToken(t Token) error {
 		p.WriteString("<!--")
 		p.Write(t)
 		p.WriteString("-->")
-		return p.cachedWriteError()
 	case ProcInst:
 		if t.Target == "xml" || !isNameString(t.Target) {
 			return fmt.Errorf("xml: EncodeToken of ProcInst with invalid Target")
@@ -219,7 +218,7 @@ func (enc *Encoder) EncodeToken(t Token) error {
 		p.Write(t)
 		p.WriteString(">")
 	}
-	return p.cachedWriteError()
+	return p.Flush()
 }
 
 type printer struct {

src/pkg/encoding/xml/marshal_test.go

@@ -1076,6 +1076,15 @@ func TestMarshalWriteIOErrors(t *testing.T) {
 	}
 }
 
+func TestEncodeTokenFlush(t *testing.T) {
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+	enc.EncodeToken(StartElement{Name: Name{Local: "some-tag"}})
+	if g, w := buf.String(), "<some-tag>"; g != w {
+		t.Errorf("Encoder wrote %q, want %q", g, w)
+	}
+}
+
 func BenchmarkMarshal(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Marshal(atomValue)