Emit to StreamCodeOutput instead of CodeBuffer.

tests/compiler/dart2js/warnings_checker.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.
 
 // Test that dart2js produces the expected static type warnings to ensures that
 // the analyzer and dart2js agrees on the tests.
 
 import 'dart:async';
 import 'dart:io';
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'memory_compiler.dart';
 import 'package:compiler/src/filenames.dart';
-import 'package:compiler/src/source_file.dart';
+import 'package:compiler/src/io/source_file.dart';
 import 'package:compiler/src/source_file_provider.dart';
 import 'package:compiler/src/util/uri_extras.dart';
 import 'dart:convert';
 
 void checkWarnings(Map<String, dynamic> tests, [List<String> arguments]) {
   bool isWindows = Platform.isWindows;
   Uri script = currentDirectory.resolveUri(Platform.script);
   bool warningsMismatch = false;
   bool verbose = arguments != null && arguments.contains('-v');
   asyncTest(() => Future.forEach(tests.keys, (String test) {
@@ skipping 49 matching lines @@
             print('$uri [${lineNo+1}]: Missing static type warning.');
             print(line);
           }
         }
       }
     });
   }).then((_) {
     Expect.isFalse(warningsMismatch);
   }));
 }