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

 // 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;
 
 /**
  * A function element that represents a closure call. The signature is copied
  * from the given element.
  */
@@ skipping 487 matching lines @@
       argumentsBuffer[0] = new js.VariableUse(receiverArgumentName);
     }
 
     int indexOfLastOptionalArgumentInParameters = positionalArgumentCount - 1;
     TreeElements elements =
         compiler.enqueuer.resolution.getCachedElements(member);
 
     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 {
         int index = names.indexOf(element.name);
         if (index != -1) {
           indexOfLastOptionalArgumentInParameters = count;
           // 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)) {
           argumentsBuffer[count] = constantReference(SentinelConstant.SENTINEL);
         } else {
           Constant value = handler.initialVariableValues[element];
           if (value == null) {
             argumentsBuffer[count] = constantReference(new NullConstant());
           } else {
             if (!value.isNull()) {
@@ skipping 1637 matching lines @@
 """;
 const String HOOKS_API_USAGE = """
 // The code supports the following hooks:
 // dartPrint(message)   - if this function is defined it is called
 //                        instead of the Dart [print] method.
 // dartMainRunner(main) - if this function is defined, the Dart [main]
 //                        method will not be invoked directly.
 //                        Instead, a closure that will invoke [main] is
 //                        passed to [dartMainRunner].
 """;