Have one, long-lived map for bound functions.

src/v8natives.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file relies on the fact that the following declarations have been made
 // in runtime.js:
 // var $Object = global.Object;
 // var $Boolean = global.Boolean;
 // var $Number = global.Number;
 // var $Function = global.Function;
@@ skipping 1763 matching lines @@
     var argv = new InternalArray(bound_argc + argc);
     for (var i = 0; i < bound_argc; i++) {
       argv[i] = bindings[i + 2];
     }
     for (var j = 0; j < argc; j++) {
       argv[i++] = %_Arguments(j);
     }
     return %Apply(bindings[0], bindings[1], argv, 0, bound_argc + argc);
   };
 
-  %FunctionRemovePrototype(boundFunction);
   var new_length = 0;
-  if (%_ClassOf(this) == "Function") {
-    // Function or FunctionProxy.
-    var old_length = this.length;
-    // FunctionProxies might provide a non-UInt32 value. If so, ignore it.
-    if ((typeof old_length === "number") &&
-        ((old_length >>> 0) === old_length)) {
-      var argc = %_ArgumentsLength();
-      if (argc > 0) argc--;  // Don't count the thisArg as parameter.
-      new_length = old_length - argc;
-      if (new_length < 0) new_length = 0;
-    }
+  var old_length = this.length;
+  // FunctionProxies might provide a non-UInt32 value. If so, ignore it.
+  if ((typeof old_length === "number") &&
+      ((old_length >>> 0) === old_length)) {
+    var argc = %_ArgumentsLength();
+    if (argc > 0) argc--;  // Don't count the thisArg as parameter.
+    new_length = old_length - argc;
+    if (new_length < 0) new_length = 0;
   }
   // This runtime function finds any remaining arguments on the stack,
   // so we don't pass the arguments object.
   var result = %FunctionBindArguments(boundFunction, this,
                                       this_arg, new_length);
 
   // We already have caller and arguments properties on functions,
   // which are non-configurable. It therefore makes no sence to
   // try to redefine these as defined by the spec. The spec says
   // that bind should make these throw a TypeError if get or set
@@ skipping 49 matching lines @@
   %SetCode($Function, FunctionConstructor);
   %SetProperty($Function.prototype, "constructor", $Function, DONT_ENUM);
 
   InstallFunctions($Function.prototype, DONT_ENUM, $Array(
     "bind", FunctionBind,
     "toString", FunctionToString
   ));
 }
 
 SetUpFunction();