Report compile-time errors for conflicting overrides as specified by latest

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 26300)
+++ runtime/vm/object.h	(working copy)
@@ -1715,6 +1715,11 @@
     return kind() == RawFunction::kSetterFunction;
   }
 
+  // Returns true if this function represents an implicit setter function.
+  bool IsImplicitSetterFunction() const {
+    return kind() == RawFunction::kImplicitSetter;
+  }
+
   // Returns true if this function represents a (possibly implicit) closure
   // function.
   bool IsClosureFunction() const {