Revert changes 17962-3 which break Dartium Debug build.

runtime/vm/snapshot_test.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.
 
 import 'dart:isolate';
-import 'dart:async';
 
 class Fields {
   Fields(int i, int j) : fld1 = i, fld2 = j, fld5 = true {}
   int fld1;
   final int fld2;
   static int fld3;
   static const int fld4 = 10;
   bool fld5;
 }
 class FieldsTest {
@@ skipping 1533 matching lines @@
 void processLines() {
   port.receive((message, SendPort replyTo) {
     if (message == TERMINATION_MESSAGE) {
       assert(replyTo == null);
       port.close();
     } else {
       replyTo.send(processLine(message), null);
     }
   });
 }