Avoid generating is$Object, and fix a typo in a condition.

lib/compiler/implementation/enqueue.dart

Index: lib/compiler/implementation/enqueue.dart
===================================================================
--- lib/compiler/implementation/enqueue.dart	(revision 12903)
+++ lib/compiler/implementation/enqueue.dart	(working copy)
@@ -264,7 +264,7 @@
           // assignments to fields.
           // TODO(ngeoffray): This should really move to the backend.
           cls.localMembers.forEach((Element member) {
-            if (!member.isInstanceMember() && !member.isField()) return;
+            if (!member.isInstanceMember() || !member.isField()) return;
             DartType type = member.computeType(compiler);
             registerIsCheck(type);
             SourceString helper = compiler.backend.getCheckedModeHelper(type);