Emit to StreamCodeOutput instead of CodeBuffer.

tests/try/poi/library_updater_test.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.
 
 // Test of LibraryUpdater (one compilation unit per library).
 library trydart.library_updater_test;
 
 import 'package:dart2js_incremental/library_updater.dart' show
     IncrementalCompilerContext,
     LibraryUpdater,
     Update;
 
 import 'package:compiler/src/dart2jslib.dart' show
     Script;
 
-import 'package:compiler/src/source_file.dart' show
+import 'package:compiler/src/io/source_file.dart' show
     StringSourceFile;
 
 import 'compiler_test_case.dart';
 
 void nolog(_) {}
 
 Script newScriptFrom(LibraryElement library, String newSource) {
   Script script = library.entryCompilationUnit.script;
   return script.copyWithFile(
       new StringSourceFile(script.file.filename, newSource));
@@ skipping 104 matching lines @@
           new LibraryUpdaterTestCase(
               before: 'main() => null;',
               after: 'main() {}',
               canReuse: true,
               updates: ['main']),
 
           // TODO(ahe): When supporting class members, test abstract methods.
       ]
   );
 }