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

mojo/public/dart/src/codec.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 bindings;
 
 int align(int size) => size + (kAlignment - (size % kAlignment)) % kAlignment;
 
 const int kAlignment = 8;
 const int kSerializedHandleSize = 4;
@@ skipping 162 matching lines @@
       _base + offset, value, Endianness.LITTLE_ENDIAN);
 
   void encodeDouble(double value, int offset) => _buffer.buffer.setFloat64(
       _base + offset, value, Endianness.LITTLE_ENDIAN);
 
   void encodeHandle(core.MojoHandle value, int offset, bool nullable) {
     if ((value == null) || !value.isValid) {
       encodeInvalideHandle(offset, nullable);
     } else {
       encodeUint32(_buffer.handles.length, offset);
-      _buffer.handles.add(value);
+      _buffer.handles.add(value.pass());
     }
   }
 
   void encodeMessagePipeHandle(
           core.MojoMessagePipeEndpoint value, int offset, bool nullable) =>
       encodeHandle(value != null ? value.handle : null, offset, nullable);
 
   void encodeConsumerHandle(
           core.MojoDataPipeConsumer value, int offset, bool nullable) =>
       encodeHandle(value != null ? value.handle : null, offset, nullable);
@@ skipping 364 matching lines @@
   bool decodeBool(int offset, int bit) =>
       (decodeUint8(offset) & (1 << bit)) != 0;
 
   core.MojoHandle decodeHandle(int offset, bool nullable) {
     int index = decodeInt32(offset);
     if (index == -1) {
       if (!nullable) {
         throw new MojoCodecError(
             'Trying to decode an invalid handle from a non-nullable type.');
       }
-      return new core.MojoHandle(core.MojoHandle.INVALID);
+      return new core.MojoHandle.invalid();
     }
     _validator.claimHandle(index);
     return _handles[index];
   }
 
   core.MojoMessagePipeEndpoint decodeMessagePipeHandle(
           int offset, bool nullable) =>
       new core.MojoMessagePipeEndpoint(decodeHandle(offset, nullable));
 
   core.MojoDataPipeConsumer decodeConsumerHandle(int offset, bool nullable) =>
@@ skipping 245 matching lines @@
       throw new MojoCodecError(
           'Incorrect header for map. The size is incorrect.');
     }
     if (header.version != kMapStructHeader.version) {
       throw new MojoCodecError(
           'Incorrect header for map. The version is incorrect.');
     }
     return header;
   }
 }