Make sure isolate library is initialized before using isolate natives.

tests/compiler/dart2js_native/compute_this_script_test.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.
 
 // Test of IsolateNatives.computeThisScript().
 
 import 'dart:_isolate_helper';
+// The isolate helper library is not correctly set up if the dart:isolate
+// library hasn't been loaded.
+import 'dart:isolate';
 
 main() {
+  // Need to use the isolate-library so dart2js correctly initializes the
+  // library.
+  Capability cab = new Capability();
   String script = IsolateNatives.computeThisScript();
 
   // This is somewhat brittle and relies on an implementation detail
   // of our test runner, but I can think of no other way to test this.
   // -- ahe
   if (!script.endsWith('/out.js')) {
     throw 'Unexpected script: "$script"';
   }
 }