Fix DisposeCapability leak (#194)

* Add NOTE to DisposeResources to indicate disposeCapability can be freed

* Add Asserts and set stack to empty on dispose

* Minor tweak to NOTE text

* Use `undefined` instead of `~empty~`

* Switch to empty list

* Minor cleanup

* Update text of NOTE

spec.emu

@@ -1117,6 +1117,8 @@ contributors: Ron Buckton, Ecma International
               1. Set _completion_ to ThrowCompletion(_error_).
             1. Else,
               1. Set _completion_ to _result_.
+        1. NOTE: After _disposeCapability_ has been disposed, it will never be used again. The contents of _disposeCapability_.[[DisposableResourceStack]] can be discarded in implementations, such as by garbage collection, at this point.
+        1. Set _disposeCapability_.[[DisposableResourceStack]] to a new empty List.
         1. Return _completion_.
       </emu-alg>
     </emu-clause>