Remove old isolate API and update all code in the repository to use

tests/isolate/spawn_test.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("SpawnTest");
-#import("dart:isolate");
-#import('../../lib/unittest/unittest.dart');
-
-main() {
-  test("spawn a new isolate", () {
-    SpawnedIsolate isolate = new SpawnedIsolate();
-    isolate.spawn().then(expectAsync1((SendPort port) {
-      port.call(42).then(expectAsync1((message) {
-        Expect.equals(42, message);
-      }));
-    }));
-  });
-}
-
-class SpawnedIsolate extends Isolate {
-
-  SpawnedIsolate() : super() { }
-
-  void main() {
-    this.port.receive((message, SendPort replyTo) {
-      Expect.equals(42, message);
-      replyTo.send(42, null);
-      this.port.close();
-    });
-  }
-
-}