fixes #201, add support for custom URL mapping

test/checker/checker_test.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.
 
 /// General type checking tests
 library dev_compiler.test.checker_test;
 
 import 'package:test/test.dart';
 
 import 'package:dev_compiler/src/testing.dart';
+import '../test_util.dart' show testDirectory;
 
 void main() {
   test('ternary operator', () {
     testChecker({
       '/main.dart': '''
         abstract class Comparable<T> {
           int compareTo(T other);
           static int compare(Comparable a, Comparable b) => a.compareTo(b);
         }
         typedef int Comparator<T>(T a, T b);
@@ skipping 2867 matching lines @@
 
           typedef I2D(int x);
           typedef D2D(x);
           typedef II2D(int x, int y);
           typedef DI2D(x, int y);
           typedef ID2D(int x, y);
           typedef DD2D(x, y);
 
           int foo(int x) => x;
           int bar(int x, int y) => x + y;
-          
+
           void main() {
             bool b;
             b = /*severe:InvalidRuntimeCheckError*/foo is I2I;
             b = /*severe:InvalidRuntimeCheckError*/foo is D2I;
             b = /*severe:InvalidRuntimeCheckError*/foo is I2D;
             b = foo is D2D;
 
             b = /*severe:InvalidRuntimeCheckError*/bar is II2I;
             b = /*severe:InvalidRuntimeCheckError*/bar is DI2I;
             b = /*severe:InvalidRuntimeCheckError*/bar is ID2I;
@@ skipping 15 matching lines @@
             f = bar as ID2I;
             f = bar as II2D;
             f = bar as DD2I;
             f = bar as DI2D;
             f = bar as ID2D;
             f = bar as DD2D;
           }
       '''
     });
   });
+
+  test('custom URL mappings', () => testChecker({
+    '/main.dart': '''
+      import 'dart:foobar' show Baz;
+      main() {
+        print(Baz.quux);
+      }'''
+  },
+      customUrlMappings: {
+    'dart:foobar': '$testDirectory/checker/dart_foobar.dart'
+  }));
 }