Avoid Element-only assertions in generateParameterStubs

pkg/compiler/lib/src/js_emitter/parameter_stub_generator.dart

 // Copyright (c) 2015, 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.
 
 library dart2js.js_emitter.parameter_stub_generator;
 
 import '../closure.dart' show ClosureClassElement;
 import '../common.dart';
 import '../common_elements.dart';
 import '../constants/values.dart';
@@ skipping 199 matching lines @@
   // (5) No stub generated, call is direct.
   //
   // We need to pay attention if this stub is for a function that has been
   // invoked from a subclass. Then we cannot just redirect, since that
   // would invoke the methods of the subclass. We have to compile to:
   // (1) foo$2(a, b) => MyClass.foo$4$c$d.call(this, a, b, null, null)
   // (2) foo$3$c(a, b, c) => MyClass.foo$4$c$d(this, a, b, c, null);
   // (3) foo$3$d(a, b, d) => MyClass.foo$4$c$d(this, a, b, null, d);
   List<ParameterStubMethod> generateParameterStubs(FunctionEntity member,
       {bool canTearOff: true}) {
-    if (member.enclosingClass != null && member.enclosingClass.isClosure) {
-      ClosureClassElement cls = member.enclosingClass;
-      if (cls.supertype.element == _commonElements.boundClosureClass) {
-        failedAt(cls.methodElement, 'Bound closure1.');
-      }
-      if (cls.methodElement.isInstanceMember) {
-        failedAt(cls.methodElement, 'Bound closure2.');
+    {
+      var enclosingClass = member.enclosingClass;
+      if (enclosingClass != null && enclosingClass.isClosure) {
+        if (enclosingClass is ClosureClassElement) {
+          // TODO(redemption): What is the equivalent invariant for J-entities?
+          var methodElement = enclosingClass.methodElement;
+          if (enclosingClass.supertype.element ==
+              _commonElements.boundClosureClass) {
+            failedAt(methodElement, 'Bound closure1.');
+          }
+          if (methodElement.isInstanceMember) {
+            failedAt(methodElement, 'Bound closure2.');
+          }
+        }
       }
     }
 
     // The set of selectors that apply to `member`. For example, for
     // a member `foo(x, [y])` the following selectors may apply:
     // `foo(x)`, and `foo(x, y)`.
     Map<Selector, SelectorConstraints> selectors;
     // The set of selectors that apply to `member` if it's name was `call`.
     // This happens when a member is torn off. In that case calls to the
     // function use the name `call`, and we must be able to handle every
@@ skipping 68 matching lines @@
             generateParameterStub(member, selector, null);
         if (stub != null) {
           stubs.add(stub);
         }
       }
     }
 
     return stubs;
   }
 }