Add writeString method to OutputStream

runtime/bin/stream_util.dart

Index: runtime/bin/stream_util.dart
diff --git a/runtime/bin/stream_util.dart b/runtime/bin/stream_util.dart
index d558bfaabb37cd0838a0f348412058c02d930cd3..f00c4a2e54800d7d0089ffbfc7f19e67a0658108 100644
--- a/runtime/bin/stream_util.dart
+++ b/runtime/bin/stream_util.dart
@@ -175,3 +175,15 @@ void _pipe(InputStream input, OutputStream output, [bool close]) {
   output.onNoPendingWrites = null;
 }
 
+
+class _BaseOutputStream {
+  bool writeString(String string, [Encoding encoding = Encoding.UTF_8]) {
+    if (string.length > 0) {
+      // Encode and write data.
+      StringEncoder encoder = _StringEncoders.encoder(encoding);
+      List<int> data = encoder.encodeString(string);
+      return write(data, copyBuffer: false);
+    }
+    return true;
+  }
+}