Ignore return value of [Symbol.dispose]() method when hint is 'async-…

…dispose' (#180)

* Ignore return value of [Symbol.dispose]() method when hint is 'async-dispose'

* Apply suggestions from PR

spec.emu

@@ -1068,6 +1068,12 @@ contributors: Ron Buckton, Ecma International
           1. Let _method_ be ? GetMethod(_V_, @@asyncDispose).
           1. If _method_ is *undefined*, then
             1. Set _method_ to ? GetMethod(_V_, @@dispose).
+            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. Return CreateBuiltinFunction(_closure_, 0, *""*, « »).
         1. Else,
           1. Let _method_ be ? GetMethod(_V_, @@dispose).
         1. Return _method_.