use a WeakMap when possible in the WebIDL cache. Helps kripken/ammo.j…

…s#284

tools/webidl_binder.py

@@ -88,7 +88,9 @@ def build_constructor(name):
   return [r'''{name}.prototype = Object.create({implementing}.prototype);
 {name}.prototype.constructor = {name};
 {name}.prototype.__class__ = {name};
-{name}.__cache__ = {{}};
+// If WeakMaps are supported, use them. This prevents the cache from rooting
+// objects and keeping them alive forever.
+{name}.__cache__ = typeof WeakMap !== 'undefined' ? new WeakMap() : {{}};
 Module['{name}'] = {name};
 '''.format(name=name, implementing=implementing_name)]