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

pkg/compiler/lib/src/dart_backend/copy_propagator.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 copy_propagator;
 
 import '../elements/elements.dart';
 import 'tree_ir_nodes.dart';
 
 /// Eliminates moving assignments, such as w := v, by assigning directly to w
@@ skipping 179 matching lines @@
     node.next = visitStatement(node.next);
     visitExpression(node.expression);
     return node;
   }
 
   void visitFunctionExpression(FunctionExpression node) {
     new CopyPropagator().rewrite(node.definition);
   }
 
 }