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

pkg/compiler/lib/src/io/source_map_builder.dart

Index: pkg/compiler/lib/src/io/source_map_builder.dart
diff --git a/pkg/compiler/lib/src/source_map_builder.dart b/pkg/compiler/lib/src/io/source_map_builder.dart
similarity index 97%
rename from pkg/compiler/lib/src/source_map_builder.dart
rename to pkg/compiler/lib/src/io/source_map_builder.dart
index b486a1ca602a68db20af1cde245fd19a40ded6d1..de80533c4c5a1f316f4dcf3229072fd595859b02 100644
--- a/pkg/compiler/lib/src/source_map_builder.dart
+++ b/pkg/compiler/lib/src/io/source_map_builder.dart
@@ -2,12 +2,12 @@
 // 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 source_map_builder;
+library dart2js.source_map_builder;
 
-import 'util/util.dart';
-import 'scanner/scannerlib.dart' show Token;
-import 'source_file.dart';
-import 'util/uri_extras.dart' show relativize;
+import '../util/util.dart';
+import '../scanner/scannerlib.dart' show Token;
+import '../source_file.dart';
+import '../util/uri_extras.dart' show relativize;
 
 class SourceMapBuilder {
   static const int VLQ_BASE_SHIFT = 5;