Add proper API for creating private symbols wrt a library.

tests/lib/mirrors/other_library.dart

Index: tests/lib/mirrors/other_library.dart
diff --git a/tests/lib/mirrors/other_library.dart b/tests/lib/mirrors/other_library.dart
new file mode 100644
index 0000000000000000000000000000000000000000..c1e908f4a8dbc1c61b5d632ed01e49a354c7fa65
--- /dev/null
+++ b/tests/lib/mirrors/other_library.dart
@@ -0,0 +1,15 @@
+// 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.
+
+library test.other_library;
+
+topLevelMethod() => 42;
+get topLevelGetter => 42;
+set topLevelSetter(x) => 42;
+var topLevelField = 42;
+
+_topLevelMethod() => 42;
+get _topLevelGetter => 42;
+set _topLevelSetter(x) => 42;
+var _topLevelField = 42;