Add option for outputting a deferred loading mapping.

pkg/compiler/lib/src/js_backend/backend.dart

 // Copyright (c) 2012, 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 js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 const bool USE_CPS_IR = const bool.fromEnvironment("USE_CPS_IR");
 
@@ skipping 2279 matching lines @@
     aliasedSuperMembers.remove(element);
   }
 
   void registerMainHasArguments(Enqueuer enqueuer) {
     // If the main method takes arguments, this compilation could be the target
     // of Isolate.spawnUri. Strictly speaking, that can happen also if main
     // takes no arguments, but in this case the spawned isolate can't
     // communicate with the spawning isolate.
     enqueuer.enableIsolateSupport();
   }
+
+  /// Returns the filename for the output-unit named [name].
+  ///
+  /// The filename is of the form "<main output file>_<name>.part.js".
+  /// If [addExtension] is false, the ".part.js" suffix is left out.
+  String deferredPartFileName(String name, {bool addExtension: true}) {
+    assert(name != "");
+    String outPath = compiler.outputUri != null
+        ? compiler.outputUri.path
+        : "out";
+    String outName = outPath.substring(outPath.lastIndexOf('/') + 1);
+    String extension = addExtension ? ".part.js" : "";
+    return "${outName}_$name$extension";
+  }
 }
 
 class JavaScriptResolutionCallbacks extends ResolutionCallbacks {
   final JavaScriptBackend backend;
 
   JavaScriptResolutionCallbacks(this.backend);
 
   void registerBackendStaticInvocation(Element element, Registry registry) {
     registry.registerStaticInvocation(backend.registerBackendUse(element));
   }
@@ skipping 186 matching lines @@
   }
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final ConstantValue constant;
   final Element annotatedElement;
 
   const Dependency(this.constant, this.annotatedElement);
 }