Support deferred library prefix syntax

runtime/lib/lib_prefix.dart

+// Copyright (c) 2014, 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:async";
+import "dart:isolate";
+
+// This type corresponds to the VM-internal class LibraryPrefix.
+
+class _LibraryPrefix {
+  _load() native "LibraryPrefix_load";
+
+  loadLibrary() {
+    var completer = new Completer<bool>();
+    var port = new RawReceivePort();
+    port.handler = (_) {
+      this._load();
+      completer.complete(true);
+      port.close();
+    };
+    port.sendPort.send(1);
+    return completer.future;
+  }
+}