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

pkg/compiler/lib/src/js_backend/native_emitter.dart

 // Copyright (c) 2012, 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.
 
 part of js_backend;
 
 class NativeEmitter {
 
   final Map<Element, ClassBuilder> cachedBuilders;
 
@@ skipping 452 matching lines @@
     // through them and add them using defineProperty.
     if (!objectProperties.isEmpty) {
       jsAst.Expression init = js(r'''
           (function(table) {
             for(var key in table)
               #(Object.prototype, key, table[key]);
            })(#)''',
           [ defPropFunction,
             new jsAst.ObjectInitializer(objectProperties)]);
 
-      if (emitterTask.compiler.enableMinification) targetBuffer.add(';');
-      targetBuffer.add(jsAst.prettyPrint(
+      if (emitterTask.compiler.enableMinification) targetBuffer.write(';');
+      targetBuffer.write(jsAst.prettyPrint(
           new jsAst.ExpressionStatement(init), compiler));
-      targetBuffer.add('\n');
+      targetBuffer.write('\n');
     }
 
-    targetBuffer.add(nativeBuffer);
-    targetBuffer.add('\n');
+    targetBuffer.write(nativeBuffer);
+    targetBuffer.write('\n');
   }
 }