dart2js: Create program_builder directory and move corresponding files.

pkg/compiler/lib/src/js_emitter/program_builder/registry.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.js_emitter.program_builder;
 
 /// Maps [LibraryElement]s to their [Element]s.
 ///
 /// Fundamentally, this class nicely encapsulates a
 /// `Map<LibraryElement, List<Element>>`.
@@ skipping 101 matching lines @@
   void registerConstant(OutputUnit outputUnit, ConstantValue constantValue) {
     // Ignore for now.
   }
 
   Holder registerHolder(String name) {
     return _holdersMap.putIfAbsent(
         name,
         () => new Holder(name, _holdersMap.length));
   }
 }