Move dart2js from sdk/lib/_internal/compiler to pkg/compiler

pkg/compiler/lib/src/dart_backend/outputter.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.
 
 part of dart_backend;
 
 typedef bool IsSafeToRemoveTypeDeclarations(
     Map<ClassElement, Iterable<Element>> classMembers);
 typedef void ElementCallback<E>(E element);
 typedef void ElementPostProcessFunction(
@@ skipping 542 matching lines @@
       outputProvider("", "dart")
            ..add(assembledCode)
            ..close();
 
       totalSize = assembledCode.length;
     }
 
     return assembledCode;
   }
 }