Add test of unittests.

tests/compiler/dart2js/source_map_validator_helper.dart

 // Copyright (c) 2014, 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.
 
 import 'dart:io';
 import 'dart:convert';
 import 'dart:async';
 
 import 'package:path/path.dart' as path;
 import 'package:expect/expect.dart';
-import 'package:source_maps/source_maps.dart' hide SourceFile;
+import 'package:source_maps/source_maps.dart';
 import 'package:compiler/src/apiimpl.dart';
 import 'package:compiler/src/elements/elements.dart'
     show AstElement,
          ClassElement,
          CompilationUnitElement,
          Element,
          FunctionElement,
          LibraryElement,
          MemberElement;
 import 'package:compiler/src/io/source_file.dart' show SourceFile;
@@ skipping 299 matching lines @@
   final Position end;
 
   Interval(this.begin, this.end);
 
   bool contains(Position other) {
     return begin <= other && other <= end;
   }
 
   String toString() => '$begin-$end';
 }