Implement modular compilation

lib/src/compiler/js_field_storage.dart

 // Copyright (c) 2015, 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.
 
-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.
 HashSet<FieldElement> findFieldsNeedingStorage(
     Iterable<CompilationUnitElement> units, ExtensionTypeSet extensionTypes) {
   var overrides = new HashSet<FieldElement>();
   for (var unit in units) {
     for (var cls in unit.types) {
       var superclasses = getSuperclasses(cls);
       for (var field in cls.fields) {
@@ skipping 62 matching lines @@
       if (mixin != null) result.add(mixin);
     }
     var supertype = cls.supertype;
     if (supertype == null) break;
 
     cls = supertype.element;
     result.add(cls);
   }
   return result;
 }