Add option for outputting a deferred loading mapping.

pkg/compiler/lib/src/js_emitter/program_builder.dart

Index: pkg/compiler/lib/src/js_emitter/program_builder.dart
diff --git a/pkg/compiler/lib/src/js_emitter/program_builder.dart b/pkg/compiler/lib/src/js_emitter/program_builder.dart
index ea6dab875c7a1c426775619f9aacf1180c0fb59e..1163ec56dab156abd0fab139bea0131197ff2c06 100644
--- a/pkg/compiler/lib/src/js_emitter/program_builder.dart
+++ b/pkg/compiler/lib/src/js_emitter/program_builder.dart
@@ -126,20 +126,11 @@ class ProgramBuilder {
     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),