Add proper API for creating private symbols wrt a library.

runtime/lib/mirrors_patch.dart

 // Copyright (c) 2012, 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:nativewrappers";
 // TODO(ahe): Move _symbol_dev.Symbol to its own "private" library?
 import "dart:_collection-dev" as _symbol_dev;
 
 /**
  * Returns a [MirrorSystem] for the current isolate.
@@ skipping 25 matching lines @@
  * Type object.
  *
  * This only works with objects local to the current isolate.
  */
 patch ClassMirror reflectClass(Type key) {
   return _Mirrors.reflectClass(key);
 }
 
 patch class MirrorSystem {
   /* patch */ static String getName(Symbol symbol) {
-    return _symbol_dev.Symbol.getName(symbol);
+    return _unmangleName(_symbol_dev.Symbol.getName(symbol));
   }
+  /* patch */ static Symbol getSymbol(String name, [LibraryMirror lib]) {
+    if (lib != null) name = _mangleName(name, lib._reflectee);
+    return new _symbol_dev.Symbol.unvalidated(name);
+  }
+
+  static _unmangleName(String name)
+      native "Mirrors_unmangleName";
+  static _mangleName(String name, _MirrorReference lib)
+      native "Mirrors_mangleName";
 }