Dart: Better handle leak checks. close() is async.

mojo/public/dart/src/data_pipe.dart

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 part of core;
 
 class _MojoDataPipeNatives {
   static List MojoCreateDataPipe(int elementBytes, int capacityBytes,
       int flags) native "MojoDataPipe_Create";
 
@@ skipping 23 matching lines @@
   MojoHandle handle;
   MojoResult status;
   final int elementBytes;
 
   MojoDataPipeProducer(this.handle,
       [this.status = MojoResult.OK, this.elementBytes = 1]);
 
   int write(ByteData data, [int numBytes = -1, int flags = 0]) {
     if (handle == null) {
       status = MojoResult.INVALID_ARGUMENT;
-      return status;
+      return 0;
     }
 
     int data_numBytes = (numBytes == -1) ? data.lengthInBytes : numBytes;
     List result = _MojoDataPipeNatives.MojoWriteData(
         handle.h, data, data_numBytes, flags);
     if (result == null) {
       status = MojoResult.INVALID_ARGUMENT;
-      return status;
+      return 0;
     }
 
     assert((result is List) && (result.length == 2));
     status = new MojoResult(result[0]);
     return result[1];
   }
 
   ByteData beginWrite(int bufferBytes, [int flags = 0]) {
     if (handle == null) {
       status = MojoResult.INVALID_ARGUMENT;
@@ skipping 118 matching lines @@
     MojoHandle consumerHandle = new MojoHandle(result[2]);
     MojoDataPipe pipe = new MojoDataPipe._internal();
     pipe.producer = new MojoDataPipeProducer(
         producerHandle, new MojoResult(result[0]), elementBytes);
     pipe.consumer = new MojoDataPipeConsumer(
         consumerHandle, new MojoResult(result[0]), elementBytes);
     pipe.status = new MojoResult(result[0]);
     return pipe;
   }
 }