Add CodeOutput to prepare for emission without in-memory buffers.

pkg/compiler/lib/src/js_emitter/js_emitter.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 dart2js.js_emitter;
 
 import '../common.dart';
 
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 
 import '../closure.dart' show
     ClosureClassElement,
     ClosureClassMap,
     ClosureFieldElement,
     CapturedVariable;
 
 import '../dart_types.dart' show
     TypedefType;
 
-import '../dart2jslib.dart' show
+import '../io/code_output.dart' show
     CodeBuffer;
 
 import '../elements/elements.dart' show
     ConstructorBodyElement,
     ElementKind,
     ParameterElement,
     TypeVariableElement;
 
 import '../hash/sha1.dart' show hashOfString;
 
@@ skipping 19 matching lines @@
 
 import 'model.dart';
 import 'program_builder.dart';
 
 import 'new_emitter/emitter.dart' as new_js_emitter;
 
 import '../source_file.dart' show
     SourceFile,
     StringSourceFile;
 
-import '../source_map_builder.dart' show
+import '../io/source_map_builder.dart' show
     SourceMapBuilder;
 
 import '../util/characters.dart' show
     $$,
     $A,
     $HASH,
     $PERIOD,
     $Z,
     $a,
     $z;
@@ skipping 27 matching lines @@
 part 'old_emitter/class_emitter.dart';
 part 'old_emitter/code_emitter_helper.dart';
 part 'old_emitter/container_builder.dart';
 part 'old_emitter/declarations.dart';
 part 'old_emitter/emitter.dart';
 part 'old_emitter/interceptor_emitter.dart';
 part 'old_emitter/metadata_emitter.dart';
 part 'old_emitter/nsm_emitter.dart';
 part 'old_emitter/reflection_data_parser.dart';
 part 'old_emitter/type_test_emitter.dart';