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

pkg/compiler/lib/src/dart_backend/statement_rewriter.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 statement_rewriter;
 
 import 'tree_ir_nodes.dart';
 
 /**
  * Performs the following transformations on the tree:
@@ skipping 533 matching lines @@
   }
 
   Expression makeCondition(Expression e, bool polarity) {
     return polarity ? e : new Not(e);
   }
 
   Statement getBranch(If node, bool polarity) {
     return polarity ? node.thenStatement : node.elseStatement;
   }
 }