isolates refactor: this change introduces 'dart:isolate' as a library. This is a

tests/isolate/src/IsolateImportNegativeTest.dart

 // Copyright (c) 2011, 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('IsolateImportNegativeTest');
+// Ommiting the following import is an error:
+// #import('dart:isolate');
 
-// A test that runs forever.
-void main() {
-  new ReceivePort().receive((x, y) => null);
-  // Does not close the receive port.
+class A extends Isolate {
+  A() : super();
+
+  void main() {}
 }
+
+main() {
+  new A();
+}
+