dart2js: Rename metadataEmitter to metadataCollector and make it independent of old emitter.

pkg/dart2js_incremental/lib/library_updater.dart

 // Copyright (c) 2014, 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.
 
 library dart2js_incremental.library_updater;
 
 import 'dart:async' show
     Future;
 
 import 'dart:convert' show
@@ skipping 136 matching lines @@
       throw new StateError("Can't set [incrementalCompiler] more than once.");
     }
     super.incrementalCompiler = value;
   }
 
   void registerUriWithUpdates(Iterable<Uri> uris) {
     _uriWithUpdates.addAll(uris);
   }
 
   void _captureState(Compiler compiler) {
-    _emittedClasses = new Set.from(compiler.backend.emitter.neededClasses);
+    JavaScriptBackend backend = compiler.backend;
+    _emittedClasses = new Set.from(backend.emitter.neededClasses);
 
     _directlyInstantiatedClasses =
         new Set.from(compiler.codegenWorld.directlyInstantiatedClasses);
 
     List<ConstantValue> constants =
-        compiler.backend.emitter.outputConstantLists[
+        backend.emitter.outputConstantLists[
             compiler.deferredLoadTask.mainOutputUnit];
     if (constants == null) constants = <ConstantValue>[];
     _compiledConstants = new Set<ConstantValue>.identity()..addAll(constants);
   }
 
   bool _uriHasUpdate(Uri uri) => _uriWithUpdates.contains(uri);
 }
 
 class LibraryUpdater extends JsFeatures {
   final Compiler compiler;
@@ skipping 1307 matching lines @@
         .buildFieldsHackForIncrementalCompilation(classElement);
     // TODO(ahe): Rewrite for new emitter.
     ClassBuilder builder = new ClassBuilder(classElement, namer);
     classEmitter.emitFields(cls, builder);
     return builder.fields;
   }
 }
 
 // TODO(ahe): Remove this method.
 NO_WARN(x) => x;