Implement modular compilation

lib/src/compiler/js_field_storage.dart

Index: lib/src/compiler/js_field_storage.dart
diff --git a/lib/src/codegen/js_field_storage.dart b/lib/src/compiler/js_field_storage.dart
similarity index 96%
rename from lib/src/codegen/js_field_storage.dart
rename to lib/src/compiler/js_field_storage.dart
index 88de7584d06e808c49827c0122d7ae7325909c32..1a70229d68fd707575d61d240f87916191f5791f 100644
--- a/lib/src/codegen/js_field_storage.dart
+++ b/lib/src/compiler/js_field_storage.dart
@@ -2,11 +2,10 @@
 // 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.
 
-import 'dart:collection' show HashMap, HashSet;
+import 'dart:collection' show HashSet;
 import 'package:analyzer/dart/ast/ast.dart' show Identifier;
 import 'package:analyzer/dart/element/element.dart';
-
-import 'js_codegen.dart' show ExtensionTypeSet;
+import 'extension_types.dart';
 
 /// We use a storage slot for fields that override or can be overridden by
 /// getter/setter pairs.