dart2js: Create program_builder directory and move corresponding files.

pkg/compiler/lib/src/js_emitter/program_builder/program_builder.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 dart2js.js_emitter.program_builder;
 
-import 'js_emitter.dart' show computeMixinClass;
-import 'model.dart';
+import '../js_emitter.dart' show computeMixinClass;
+import '../model.dart';
 
-import '../common.dart';
-import '../js/js.dart' as js;
+import '../../common.dart';
+import '../../js/js.dart' as js;
 
-import '../js_backend/js_backend.dart' show
+import '../../js_backend/js_backend.dart' show
     Namer,
     JavaScriptBackend,
     JavaScriptConstantCompiler;
 
-import 'js_emitter.dart' show
+import '../js_emitter.dart' show
     ClassStubGenerator,
     CodeEmitterTask,
     InterceptorStubGenerator,
     MainCallStubGenerator,
     ParameterStubGenerator,
     RuntimeTypeGenerator,
     TypeTestProperties;
 
-import '../elements/elements.dart' show ParameterElement, MethodElement;
+import '../../elements/elements.dart' show ParameterElement, MethodElement;
 
-import '../universe/universe.dart' show Universe, TypeMaskSet;
-import '../deferred_load.dart' show DeferredLoadTask, OutputUnit;
+import '../../universe/universe.dart' show Universe, TypeMaskSet;
+import '../../deferred_load.dart' show DeferredLoadTask, OutputUnit;
 
 part 'registry.dart';
 
+/// Builds a self-contained representation of the program that can then be
+/// emitted more easily by the individual emitters.
 class ProgramBuilder {
   final Compiler _compiler;
   final Namer namer;
   final CodeEmitterTask _task;
 
   final Registry _registry;
 
   /// True if the program should store function types in the metadata.
   bool _storeFunctionTypesInMetadata = false;
 
@@ skipping 716 matching lines @@
       _registry.registerConstant(outputUnit, constantValue);
       assert(!_constants.containsKey(constantValue));
       js.Name name = namer.constantName(constantValue);
       String constantObject = namer.globalObjectForConstant(constantValue);
       Holder holder = _registry.registerHolder(constantObject);
       Constant constant = new Constant(name, holder, constantValue);
       _constants[constantValue] = constant;
     }
   }
 }