Fix crash due to wrong position of an optional parameter in a stub method of an interceptor class.

sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart	(revision 16911)
+++ sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart	(working copy)
@@ -505,7 +505,8 @@
     parameters.orderedForEachParameter((Element element) {
       String jsName = JsNames.getValid(element.name.slowToString());
       assert(jsName != receiverArgumentName);
-      if (count < positionalArgumentCount + extraArgumentCount) {
+      int optionalParameterStart = positionalArgumentCount + extraArgumentCount;
+      if (count < optionalParameterStart) {
         parametersBuffer[count] = new js.Parameter(jsName);
         argumentsBuffer[count] = new js.VariableUse(jsName);
       } else {
@@ -515,7 +516,7 @@
           // The order of the named arguments is not the same as the
           // one in the real method (which is in Dart source order).
           argumentsBuffer[count] = new js.VariableUse(jsName);
-          parametersBuffer[selector.positionalArgumentCount + index] =
+          parametersBuffer[optionalParameterStart + index] =
               new js.Parameter(jsName);
         // Note that [elements] may be null for a synthesized [member].
         } else if (elements != null && elements.isParameterChecked(element)) {