Add proper API for creating private symbols wrt a library.

tests/lib/mirrors/intercepted_class_test.dart

 // Copyright (c) 2013, 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.
 
 // Ensure that classes handled specially by dart2js can be reflected on.
 
 library test.intercepted_class_test;
 
 import 'dart:mirrors';
+import 'package:expect/expect.dart';
 
 import 'stringify.dart' show stringify, expect;
 
 checkClassMirror(ClassMirror cls, String name) {
-  expect('s($name)', cls.simpleName);
+  bool foundInterface = false;
+  for(ClassMirror cm = cls; cm != null; cm = cm.superclass) {
+    if (MirrorSystem.getName(cm.simpleName) == name) foundInterface = true;
+    cm.superinterfaces.forEach((intf) {
+      if (MirrorSystem.getName(intf.simpleName) == name) foundInterface = true;
+    });
+  }
+  Expect.isTrue(foundInterface, '$cls should implement $name');
+
   var variables = new Map();
-  cls.variables.forEach((Symbol key, VariableMirror value) {
-    if (!value.isStatic && !value.isPrivate) {
-      variables[key] = value;
-    }
-  });
+  for(ClassMirror cm = cls; cm != null; cm = cm.superclass) {
+    cm.variables.forEach((Symbol key, VariableMirror value) {
+      if (!value.isStatic && !value.isPrivate) {
+        variables[key] = value;
+      }
+    });
+  }
   expect('{}', variables);
 }
 
 main() {
   checkClassMirror(reflectClass(String), 'String');
   checkClassMirror(reflectClass(int), 'int');
   checkClassMirror(reflectClass(double), 'double');
   checkClassMirror(reflectClass(num), 'num');
   checkClassMirror(reflectClass(bool), 'bool');
   checkClassMirror(reflectClass(List), 'List');
 }