Add PromiseCapability wrapper around sync dispose used in an `await u…

…sing` (#218)

spec.emu

@@ -1068,9 +1068,12 @@ contributors: Ron Buckton, Ecma International
             1. If _method_ is not *undefined*, then
               1. Let _closure_ be a new Abstract Closure with no parameters that captures _method_ and performs the following steps when called:
                 1. Let _O_ be the *this* value.
-                1. Perform ? Call(_method_, _O_).
-                1. Return *undefined*.
-              1. NOTE: This function is not observable to user code. It is used to ensure that a Promise returned from a synchronous `@@dispose` method will not be awaited.
+                1. Let _promiseCapability_ be ! NewPromiseCapability(%Promise%).
+                1. Let _result_ be Completion(Call(_method_, _O_)).
+                1. IfAbruptRejectPromise(_result_, _promiseCapability_).
+                1. Perform ? Call(_promiseCapability_.[[Resolve]], *undefined*, « *undefined* »).
+                1. Return _promiseCapability_.[[Promise]].
+              1. NOTE: This function is not observable to user code. It is used to ensure that a Promise returned from a synchronous `@@dispose` method will not be awaited and that any exception thrown will not be thrown synchronously.
               1. Return CreateBuiltinFunction(_closure_, 0, *""*, « »).
         1. Else,
           1. Let _method_ be ? GetMethod(_V_, @@dispose).