Fix standalone tests to pass dartc.

tests/standalone/debugger/debug_lib.dart

 // Copyright (c) 2013, 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.
 
 // Library used by debugger wire protocol tests (standalone VM debugging).
 
 library DartDebugger;
 
+import "dart:async";
 import "dart:io";
 import "dart:math";
 import "dart:utf";
 import "dart:json" as JSON;
 
 // Whether or not to print the debugger wire messages on the console.
 var verboseWire = false;
 
 // The number of attempts made to find an unused debugger port.
 var retries = 0;
@@ skipping 507 matching lines @@
         var trace = getAttachedStackTrace(e);
         if (trace != null) print("StackTrace: $trace");
         return -1; 
       } else {
         // Retry with another random port.
         RunScript(script);
       }
     });
   return true;
 }