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

pkg/compiler/lib/src/dart_backend/backend.dart

 // Copyright (c) 2012, 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;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
@@ skipping 430 matching lines @@
   }
 
   ConstantExpression compileMetadata(MetadataAnnotation metadata,
                            Node node,
                            TreeElements elements) {
     return measure(() {
       return constantCompiler.compileMetadata(metadata, node, elements);
     });
   }
 }