dart2js: Change typedef encoding.

sdk/lib/_internal/compiler/js_lib/shared/runtime_data.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.
-
-/// Contains encoding, decoding and detection functionality for the
-/// representation of program data at runtime.
-///
-/// This library is shared between the compiler and the runtime system.
-library dart2js.runtime_data;
-
-
-String encodeTypedefFieldDescriptor(int typeIndex) {
-  return ":$typeIndex;";
-}
-
-bool isTypedefDescriptor(String descriptor) {
-  return descriptor.startsWith(':');
-}
-
-int getTypeFromTypedef(String descriptor) {
-  return int.parse(descriptor.substring(1, descriptor.length - 1));
-}