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

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

Index: pkg/compiler/lib/src/js_backend/native_emitter.dart
diff --git a/pkg/compiler/lib/src/js_backend/native_emitter.dart b/pkg/compiler/lib/src/js_backend/native_emitter.dart
index 4d2008cb9c1b30e6167ce7d763c55ccff578f909..7876e378bc405b941e6eeb9c8f637520ca03aa99 100644
--- a/pkg/compiler/lib/src/js_backend/native_emitter.dart
+++ b/pkg/compiler/lib/src/js_backend/native_emitter.dart
@@ -470,13 +470,13 @@ class NativeEmitter {
           [ 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');
   }
 }