Rename constructors.dart to constant_constructors.dart

pkg/compiler/lib/src/constants/constant_constructors.dart

 // Copyright (c) 2015, 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 dart2js.constants.constructors;
 
 import '../dart2jslib.dart';
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../resolution/resolution.dart';
@@ skipping 357 matching lines @@
     }
     return new IdenticalConstantExpression(
         apply(arguments.nodes.head), apply(arguments.nodes.tail.head));
   }
 
   @override
   ConstantExpression visitNamedArgument(NamedArgument node) {
     return apply(node.expression);
   }
 }