dart2js: Remove oldEmitter field in emitter-task.

pkg/dart2js_incremental/lib/dart2js_incremental.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;
 
 import 'dart:async' show
     EventSink,
     Future;
 
 import 'dart:profiler' show
     UserTag;
 
 import 'package:compiler/src/apiimpl.dart' show
     Compiler;
 
 import 'package:compiler/compiler.dart' show
     CompilerInputProvider,
     CompilerOutputProvider,
     Diagnostic,
     DiagnosticHandler;
 
 import 'package:compiler/src/dart2jslib.dart' show
     NullSink;
 
 import 'package:compiler/src/js_backend/js_backend.dart' show
     JavaScriptBackend;
 
+import 'package:compiler/src/js_emitter/js_emitter.dart' show
+    OldEmitter;
+
 import 'package:compiler/src/elements/elements.dart' show
     LibraryElement;
 
 import 'library_updater.dart' show
     IncrementalCompilerContext,
     LibraryUpdater,
     Logger;
 
 import 'package:compiler/src/js/js.dart' as jsAst;
 
@@ skipping 121 matching lines @@
   }
 }
 
 class IncrementalCompilationFailed {
   final String reason;
 
   const IncrementalCompilationFailed(this.reason);
 
   String toString() => "Can't incrementally compile program.\n\n$reason";
 }