Keep function.prototype fast.

test/mjsunit/fast-prototype.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 54 matching lines @@
     // Adding this many properties makes it slow.
     assertFalse(%HasFastProperties(proto));
     DoProtoMagic(proto, set__proto__);
     // Making it a prototype makes it fast again.
     assertTrue(%HasFastProperties(proto));
   } else {
     DoProtoMagic(proto, set__proto__);
     // 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;
 }
 
 // TODO(mstarzinger): This test fails easily if gc happens at the wrong time.
 gc();
 
 for (var i = 0; i < 4; i++) {
   var set__proto__ = ((i & 1) != 0);
@@ skipping 18 matching lines @@
   assertTrue(key == 'a');
   break;
 }
 assertFalse(%HasFastProperties(x));
 x.d = 4;
 assertFalse(%HasFastProperties(x));
 for (key in x) {
   assertTrue(key == 'a');
   break;
 }