Fix initial prototype of WeakMap function.

src/weakmap.js

Index: src/weakmap.js
diff --git a/src/weakmap.js b/src/weakmap.js
index 70210b98de2d110b7781284ae20bf954d0c86993..3d261e5afda2e18502f3af41661b96cdd977258b 100644
--- a/src/weakmap.js
+++ b/src/weakmap.js
@@ -81,13 +81,16 @@ function WeakMapDelete(key) {
 // -------------------------------------------------------------------
 
 function SetupWeakMap() {
-  // Setup the WeakMap constructor function.
+  // Set up the WeakMap constructor function.
   %SetCode($WeakMap, WeakMapConstructor);
 
-  // Setup the WeakMap prototype object.
+  // Set up the WeakMap prototype object.
   %FunctionSetPrototype($WeakMap, new $WeakMap());
 
-  // Setup the non-enumerable functions on the WeakMap prototype object.
+  // Set up the constructor property on the WeakMap prototype object.
+  %SetProperty($WeakMap.prototype, "constructor", $WeakMap, DONT_ENUM);
+
+  // Set up the non-enumerable functions on the WeakMap prototype object.
   InstallFunctionsOnHiddenPrototype($WeakMap.prototype, DONT_ENUM, $Array(
     "get", WeakMapGet,
     "set", WeakMapSet,