Keep function.prototype fast.

test/mjsunit/fast-prototype.js

Index: test/mjsunit/fast-prototype.js
diff --git a/test/mjsunit/fast-prototype.js b/test/mjsunit/fast-prototype.js
index cdcc1a9ed68e8995b4642b5702166e781b46e211..55a0faae805341b16804deeef6e53e7a49dfc1cd 100644
--- a/test/mjsunit/fast-prototype.js
+++ b/test/mjsunit/fast-prototype.js
@@ -72,10 +72,15 @@ function test(use_new, add_first, set__proto__, same_map_as) {
     // Still fast
     assertTrue(%HasFastProperties(proto));
     AddProps(proto);
-    // After we add all those properties it went slow mode again :-(
-    assertFalse(%HasFastProperties(proto));
+    if (set__proto__) {
+      // After we add all those properties it went slow mode again :-(
+      assertFalse(%HasFastProperties(proto));
+    } else {
+      // .prototype keeps it fast.
+      assertTrue(%HasFastProperties(proto));
+    }
   }
-  if (same_map_as && !add_first) {
+  if (same_map_as && !add_first && set__proto__) {
     assertTrue(%HaveSameMap(same_map_as, proto));
   }
   return proto;