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

mojo/public/dart/src/message_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 _MojoMessagePipeNatives {
   static List MojoCreateMessagePipe(int flags) native "MojoMessagePipe_Create";
 
   static int MojoWriteMessage(int handle, ByteData data, int numBytes,
@@ skipping 64 matching lines @@
     }
 
     // If numBytes has the default value, use the full length of the data.
     int dataNumBytes;
     if (data == null) {
       dataNumBytes = 0;
     } else {
       dataNumBytes = (numBytes == -1) ? data.lengthInBytes : numBytes;
       if (dataNumBytes > data.lengthInBytes) {
         status = MojoResult.INVALID_ARGUMENT;
-        return status;
+        return null;
       }
     }
 
     // handles may be null, otherwise make an int list for the handles.
     List<int> mojoHandles;
     if (handles == null) {
       mojoHandles = null;
     } else {
       mojoHandles = new List<int>(handles.length);
     }
@@ skipping 53 matching lines @@
     MojoHandle end1 = new MojoHandle(result[1]);
     MojoHandle end2 = new MojoHandle(result[2]);
     MojoMessagePipe pipe = new MojoMessagePipe._();
     pipe.endpoints = new List(2);
     pipe.endpoints[0] = new MojoMessagePipeEndpoint(end1);
     pipe.endpoints[1] = new MojoMessagePipeEndpoint(end2);
     pipe.status = new MojoResult(result[0]);
     return pipe;
   }
 }