fixes #201, add support for custom URL mapping

test/checker/dart_foobar.dart

 // Copyright (c) 2015, 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 circle;
+/// This is a test for --url-mapping option.
+/// Analyzer's CustomUriResolver only supports on-disk resources, so we make
+/// this file accessible.
+library dart.foobar;
 
-class Circle {
-  final num x, y, radius;
-
-  Circle(this.x, this.y, this.radius);
+class Baz {
+  static String quux = "hello world";
 }