Runtime support for the new parameter specification.

lib/compiler/implementation/js_backend/emitter.dart

Index: lib/compiler/implementation/js_backend/emitter.dart
===================================================================
--- lib/compiler/implementation/js_backend/emitter.dart	(revision 12186)
+++ lib/compiler/implementation/js_backend/emitter.dart	(working copy)
@@ -363,6 +363,8 @@
     // The arguments that will be passed to the real method.
     List<String> argumentsBuffer = new List<String>(parameters.parameterCount);
 
+    // TODO(5074): Update this comment once we remove support for
+    // the deprecated parameter specification.
     // We fill the lists depending on the selector. For example,
     // take method foo:
     //    foo(a, b, [c, d]);