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

pkg/compiler/lib/src/compile_time_constants.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 dart2js;
 
 /// A [ConstantEnvironment] provides access for constants compiled for variable
 /// initializers.
 abstract class ConstantEnvironment {
   /// Returns the constant for the initializer of [element].
@@ skipping 1081 matching lines @@
     return new AstConstant(
         element,
         element.initializer != null ? element.initializer : element.node,
         constant);
   }
 
   ConstantValue get value => expression.value;
 
   String toString() => expression.toString();
 }