Support forEachClassMember on closure classes

pkg/compiler/lib/src/universe/member_usage.dart

 // Copyright (c) 2017, 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 world_builder;
 
 abstract class AbstractUsage<T> {
   final EnumSet<T> _pendingUse = new EnumSet<T>();
 
   AbstractUsage() {
@@ skipping 31 matching lines @@
       if (member.isAssignable) {
         return new _FieldUsage(member, isNative: isNative);
       } else {
         return new _FinalFieldUsage(member, isNative: isNative);
       }
     } else if (member.isGetter) {
       return new _GetterUsage(member);
     } else if (member.isSetter) {
       return new _SetterUsage(member);
     } else {
-      assert(member.isFunction);
+      assert(member.isFunction, failedAt(member, "Unexpected member: $member"));
       return new _FunctionUsage(member);
     }
   }
 
   /// `true` if [entity] has been read as a value. For a field this is a normal
   /// read access, for a function this is a closurization.
   bool get hasRead => false;
 
   /// `true` if a value has been written to [entity].
   bool get hasWrite => false;
@@ skipping 316 matching lines @@
     if (hasClosurization) {
       return MemberUses.NONE;
     }
     hasNormalUse = hasClosurization = true;
     return _pendingUse.removeAll(MemberUses.ALL_STATIC);
   }
 
   @override
   EnumSet<MemberUse> get _originalUse => MemberUses.ALL_STATIC;
 }