Adding support for `sourceRoot` to the SingleMapping class.

pkg/source_maps/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 82 matching lines @@
     expect(map.lines.first.entries.length, 1);
     TargetEntry entry = map.lines.first.entries.first;
 
     expect(entry.sourceUrlId, 0);
     expect(entry.sourceUrlId, 0);
     expect(entry.sourceColumn, 0);
     expect(entry.sourceLine, 0);
     expect(entry.sourceNameId, 0);
   });
 
+  test('parse with source root', () {
+    var inputMap = new Map.from(MAP_WITH_SOURCE_LOCATION);
+    inputMap['sourceRoot'] = '/pkg/';
+    var mapping = parseJson(inputMap);
+    expect(mapping.spanFor(0, 0).sourceUrl, "/pkg/input.dart");
+
+    var newSourceRoot = '/new/';
+
+    mapping.sourceRoot = newSourceRoot;
+    inputMap["sourceRoot"] = newSourceRoot;
+
+    expect(mapping.toJson(), equals(inputMap));
+  });
+
   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));
     }
   });
 }