diff --git a/test/mjsunit/fast-prototype.js b/test/mjsunit/fast-prototype.js
new file mode 100644
index 0000000..9864761
--- /dev/null
+++ b/test/mjsunit/fast-prototype.js
@@ -0,0 +1,123 @@
+// 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
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// Flags: --allow-natives-syntax --expose-gc
+// Flags: --noincremental-marking
+
+// Check that objects that are used for prototypes are in the fast mode.
+
+function Super() {
+}
+
+
+function Sub() {
+}
+
+
+function AddProps(obj) {
+  for (var i = 0; i < 26; i++) {
+    obj["x" + i] = 0;
+  }
+}
+
+
+function DoProtoMagic(proto, set__proto__) {
+  if (set__proto__) {
+    (new Sub()).__proto__ = proto;
+  } else {
+    Sub.prototype = proto;
+    // Need to instantiate Sub to mark .prototype as prototype.
+    new Sub();
+  }
+}
+
+
+function test(use_new, add_first, set__proto__, same_map_as) {
+  var proto = use_new ? new Super() : {};
+
+  // New object is fast.
+  assertTrue(%HasFastProperties(proto));
+
+  if (add_first) {
+    AddProps(proto);
+    // 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);
+    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 && 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);
+  var use_new = ((i & 2) != 0);
+
+  test(use_new, true, set__proto__);
+
+  var last = test(use_new, false, set__proto__);
+  test(use_new, false, set__proto__, last);
+}
+
+
+var x = {a: 1, b: 2, c: 3};
+var o = { __proto__: x };
+assertTrue(%HasFastProperties(x));
+for (key in x) {
+  assertTrue(key == 'a');
+  break;
+}
+delete x.b;
+for (key in x) {
+  assertTrue(key == 'a');
+  break;
+}
+assertFalse(%HasFastProperties(x));
+x.d = 4;
+assertFalse(%HasFastProperties(x));
+for (key in x) {
+  assertTrue(key == 'a');
+  break;
+}