Add option for outputting a deferred loading mapping.

pkg/compiler/lib/src/js_emitter/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 '../common.dart';
@@ skipping 110 matching lines @@
         backend.emitter.staticFunctionAccess(_compiler.mainFunction),
         _buildLibraries(fragment),
         _buildStaticNonFinalFields(fragment),
         _buildStaticLazilyInitializedFields(fragment),
         _buildConstants(fragment),
         _registry.holders.toList(growable: false));
     _outputs[fragment.outputUnit] = result;
     return result;
   }
 
-  /// Returns a name composed of the main output file name and [name].
-  String _outputFileName(String name) {
-    assert(name != "");
-    String outPath = _compiler.outputUri != null
-        ? _compiler.outputUri.path
-        : "out";
-    String outName = outPath.substring(outPath.lastIndexOf('/') + 1);
-    return "${outName}_$name";
-  }
-
   DeferredOutput _buildDeferredOutput(MainOutput mainOutput,
                                       Fragment fragment) {
     DeferredOutput result = new DeferredOutput(
-        _outputFileName(fragment.name), fragment.name,
+        backend.deferredPartFileName(fragment.name, addExtension: false),
+                                     fragment.name,
         mainOutput,
         _buildLibraries(fragment),
         _buildStaticNonFinalFields(fragment),
         _buildStaticLazilyInitializedFields(fragment),
         _buildConstants(fragment));
     _outputs[fragment.outputUnit] = result;
     return result;
   }
 
   List<Constant> _buildConstants(Fragment fragment) {
@@ skipping 291 matching lines @@
     for (ConstantValue constantValue in constantValues) {
       assert(!_constants.containsKey(constantValue));
       String name = namer.constantName(constantValue);
       String constantObject = namer.globalObjectForConstant(constantValue);
       Holder holder = _registry.registerHolder(constantObject);
       Constant constant = new Constant(name, holder, constantValue);
       _constants[constantValue] = constant;
     };
   }
 }