Add proper API for creating private symbols wrt a library.

sdk/lib/mirrors/mirrors.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.
 
 // For the purposes of the mirrors library, we adopt a naming
 // convention with respect to getters and setters.  Specifically, for
 // some variable or field...
 //
 //   var myField;
 //
@@ skipping 86 matching lines @@
 
   /**
    * Returns the name of [symbol].
    *
    * The following text is non-normative:
    *
    * Using this method may result in larger output.  If possible, use
    * [MirrorsUsed] to specify which symbols must be retained in clear text.
    */
   external static String getName(Symbol symbol);
+
+  /**
+   * Returns a symbol for [name]. If [library] is not a [LibraryMirror] or if
+   * [name] is a private identifier and [library] is [:null:], throws an
+   * [ArgumentError]. If [name] is a private identifier, the symbol returned is
+   * with respect to [library].
+   *
+   * The following text is non-normative:
+   *
+   * Using this method may result in larger output.  If possible, use
+   * the const constructor of Symbol or symbol literals.
+   */
+  external static Symbol getSymbol(String name, [LibraryMirror library]);
 }
 
 /**
  * Returns a [MirrorSystem] for the current isolate.
  */
 external MirrorSystem currentMirrorSystem();
 
 /**
  * Creates a [MirrorSystem] for the isolate which is listening on
  * the [SendPort].
@@ skipping 1246 matching lines @@
    *
    * When used as metadata on an import of "dart:mirrors", this metadata does
    * not apply to the library in which the annotation is used, but instead
    * applies to the other libraries (all libraries if "*" is used).
    */
   final override;
 
   const MirrorsUsed(
       {this.symbols, this.targets, this.metaTargets, this.override});
 }