Add proper API for creating private symbols wrt a library.

tests/lib/mirrors/invoke_import_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.
+
+library test.invoke_import_test;
+
+import 'dart:mirrors';
+import 'package:expect/expect.dart';
+
+import 'other_library.dart';
+
+main() {
+  LibraryMirror thisLibrary =
+      currentMirrorSystem().findLibrary(#test.invoke_import_test).single;
+
+  Expect.throws(() => thisLibrary.invoke(#topLevelMethod, []),
+                (e) => e is NoSuchMethodError,
+                'Should not invoke imported method #topLevelMethod');
+
+  Expect.throws(() => thisLibrary.getField(#topLevelGetter),
+                (e) => e is NoSuchMethodError,
+                'Should not invoke imported getter #topLevelGetter');
+
+  Expect.throws(() => thisLibrary.getField(#topLevelField),
+                (e) => e is NoSuchMethodError,
+                'Should not invoke imported field #topLevelField');
+
+  Expect.throws(() => thisLibrary.setField(#topLevelSetter, 23),
+                (e) => e is NoSuchMethodError,
+                'Should not invoke imported setter #topLevelSetter');
+
+  Expect.throws(() => thisLibrary.setField(#topLevelField, 23),
+                (e) => e is NoSuchMethodError,
+                'Should not invoke imported field #topLevelField');
+}