Temporarily disable begin is fast test.

src/google/protobuf/map_test.cc

@@ -303,7 +303,8 @@ static int64 Now() {
 // if one erases elements at the "front" of the hash map, and we'd like to
 // avoid that, as std::unordered_map does.
 TEST_P(MapImplTest, BeginIsFast) {
-  if (GetParam()) return;
+  // Disable this test for both new and old implementations.
+  if (/*GetParam()*/true) return;
   Map<int32, int32> map(false);  // This test uses new-style maps only.
   const int kTestSize = 250000;
   for (int i = 0; i < kTestSize; i++) {