[proxy] fixing harmony/proxy.js tests and improving error messages + some drive-by fixes

src/prototype.h

Index: src/prototype.h
diff --git a/src/prototype.h b/src/prototype.h
index 2be43e4fbdc9d0794e8670938f709d77652d0387..87c3aeed8f40c70e42a9d72e9945f8363b5d0367 100644
--- a/src/prototype.h
+++ b/src/prototype.h
@@ -63,6 +63,22 @@ class PrototypeIterator {
 
   ~PrototypeIterator() {}
 
+  const bool HasAccess() {
+    Handle<JSObject> current;
+    if (handle_.is_null()) {
+      if (!object_->IsJSObject()) return true;
+      current = Handle<JSObject>(JSObject::cast(object_));
+    } else {
+      if (!handle_->IsJSObject()) return true;
+      current = Handle<JSObject>::cast(handle_);
+    }
+    if (current->IsJSGlobalProxy() || current->IsJSGlobalObject() ||
+        current->IsAccessCheckNeeded()) {
+      return isolate_->MayAccess(handle(isolate_->context()), current);
+    }
+    return true;
+  }
+
   template <typename T = Object>
   T* GetCurrent() const {
     DCHECK(handle_.is_null());