Unify String.

corelib/src/implementation/string.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.
+
+class StringImplementation {
+  /**
+   * Factory implementation of String.fromCharCodes:
+   * Allocates a new String for the specified [charCodes].
+   */
+  factory String.fromCharCodes(List<int> charCodes) {
+    return _fromCharCodes(charCodes);
+  }
+
+  external static String _fromCharCodes(List<int> charCodes);
+}