Add a mapUrl parameter to parse() and parseJson().

test/parser_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.parser_test;
 
 import 'dart:convert';
 import 'package:unittest/unittest.dart';
 import 'package:source_maps/source_maps.dart';
 import 'common.dart';
@@ skipping 96 matching lines @@
     expect(mapping.spanFor(0, 0).sourceUrl, Uri.parse("/pkg/input.dart"));
 
     var newSourceRoot = '/new/';
 
     mapping.sourceRoot = newSourceRoot;
     inputMap["sourceRoot"] = newSourceRoot;
 
     expect(mapping.toJson(), equals(inputMap));
   });
 
+  test('parse with map URL', () {
+    var inputMap = new Map.from(MAP_WITH_SOURCE_LOCATION);
+    inputMap['sourceRoot'] = 'pkg/';
+    var mapping = parseJson(inputMap, mapUrl: "file:///path/to/map");
+    expect(mapping.spanFor(0, 0).sourceUrl,
+        Uri.parse("file:///path/to/pkg/input.dart"));
+  });
+
   test('parse and re-emit', () {
     for (var expected in [
         EXPECTED_MAP,
         MAP_WITH_NO_SOURCE_LOCATION,
         MAP_WITH_SOURCE_LOCATION,
         MAP_WITH_SOURCE_LOCATION_AND_NAME]) {
       var mapping = parseJson(expected);
       expect(mapping.toJson(), equals(expected));
     }
   });
 }