Split source_mapping_test pr. test file.

tests/compiler/dart2js/sourcemaps/source_mapping_tester.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.
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'sourcemap_helper.dart';
 
 main(List<String> arguments) {
@@ skipping 99 matching lines @@
 class TestResult {
   final String config;
   final String file;
   final SourceMapProcessor processor;
   List<SourceMapInfo> userInfoList = <SourceMapInfo>[];
   Map<SourceMapInfo, Iterable<CodePoint>> failureMap =
       <SourceMapInfo, Iterable<CodePoint>>{};
 
   TestResult(this.config, this.file, this.processor);
 }