Fix GetDisposeMethod to match consensus (#217)

Author: rbuckton

spec.emu

@@ -1065,12 +1065,13 @@ 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. 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. Return CreateBuiltinFunction(_closure_, 0, *""*, « »).
         1. Else,
           1. Let _method_ be ? GetMethod(_V_, @@dispose).
         1. Return _method_.