[es6] Fix length property of collection constructors

src/weak-collection.js

Index: src/weak-collection.js
diff --git a/src/weak-collection.js b/src/weak-collection.js
index 776043d9dcdebb411cb39f92e899867f91f9cdda..03ad6ac5aeddff689bbc6977e0cad131ec0b9ff5 100644
--- a/src/weak-collection.js
+++ b/src/weak-collection.js
@@ -85,6 +85,7 @@ function SetUpWeakMap() {
   %CheckIsBootstrapping();
 
   %SetCode($WeakMap, WeakMapConstructor);
+  %FunctionSetLength($WeakMap, 0);
   %FunctionSetPrototype($WeakMap, new $Object());
   %AddNamedProperty($WeakMap.prototype, "constructor", $WeakMap, DONT_ENUM);
   %AddNamedProperty(
@@ -162,6 +163,7 @@ function SetUpWeakSet() {
   %CheckIsBootstrapping();
 
   %SetCode($WeakSet, WeakSetConstructor);
+  %FunctionSetLength($WeakSet, 0);
   %FunctionSetPrototype($WeakSet, new $Object());
   %AddNamedProperty($WeakSet.prototype, "constructor", $WeakSet, DONT_ENUM);
   %AddNamedProperty(