Add proper API for creating private symbols wrt a library.

sdk/lib/mirrors/mirrors.dart

Index: sdk/lib/mirrors/mirrors.dart
diff --git a/sdk/lib/mirrors/mirrors.dart b/sdk/lib/mirrors/mirrors.dart
index 0b9ebe52901ad6e493209ce56976a73721e9e9c7..6e21e388ef369ccbb863ef6ebf59bfff5ce70dfa 100644
--- a/sdk/lib/mirrors/mirrors.dart
+++ b/sdk/lib/mirrors/mirrors.dart
@@ -104,6 +104,17 @@ abstract class MirrorSystem {
    * [MirrorsUsed] to specify which symbols must be retained in clear text.
    */
   external static String getName(Symbol symbol);
+
+  /**
+   * Returns a symbol for [name]. If [name] is a private identifier, the symbol
+   * returned is with respect to [lib].
+   *
+   * 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 lib]);
 }
 
 /**