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

pkg/compiler/lib/src/cps_ir/constant_propagation.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 dart2js.optimizers;
 
 /**
  * Propagates constants throughout the IR, and replaces branches with fixed
  * jumps as well as side-effect free expressions with known constant results.
  * Should be followed by the [ShrinkingReducer] pass.
@@ skipping 646 matching lines @@
       return that;
     }
 
     if (this.constant == that.constant) {
       return this;
     }
 
     return NonConst;
   }
 }