Add option for outputting a deferred loading mapping.

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

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index a60666070a0cf83438a3abc8167e44f6a4fd6da8..82b738c4a498c02ddb5ce8d8e51e2cd967e01005 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -2307,6 +2307,17 @@ class JavaScriptBackend extends Backend {
     // communicate with the spawning isolate.
     enqueuer.enableIsolateSupport();
   }
+
+  /// Returns a name composed of the main output file name and [name].

[floitsch, 2015/01/05 15:46:19]

Not your comment, but:
That describes what it does, but not how it should be used.
I also don't think that it needs to do that.

Maybe:
Returns a unique filename for [name].

The given [name] should have an extension (if necessary).


That said: I guess it would be nice, if it took an optional extension argument.
(if that's how the function is used most often).

[sigurdm, 2015/01/06 09:02:20]

Done.

+  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";
+  }
+
 }
 
 class JavaScriptResolutionCallbacks extends ResolutionCallbacks {