Fix type checks that wrongly ignore malformed types.

runtime/bin/stream_util.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class _BaseDataInputStream {
   abstract int available();
 
   List<int> read([int len]) {
     if (_closeCallbackCalled) return null;
     int bytesToRead = available();
@@ skipping 162 matching lines @@
   input.onData = pipeDataHandler;
   input.onClosed = pipeCloseHandler;
   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);
+      _StringEncoder encoder = _StringEncoders.encoder(encoding);
       List<int> data = encoder.encodeString(string);
       return write(data, copyBuffer: false);
     }
     return true;
   }
 }