Allow async and includes as member names (#769)

* Allow `async` and `includes` as member names

* Allow "async" as a valid argument name

* OptionalOperationName

index.bs

@@ -640,7 +640,7 @@ in the declaration:
 *   For [=operations=], the
     <emu-t class="regex"><a href="#prod-identifier">identifier</a></emu-t> token that appears
     after the return type but before the opening parenthesis (that is,
-    one that is matched as part of the <emu-nt><a href="#prod-OptionalIdentifier">OptionalIdentifier</a></emu-nt>
+    one that is matched as part of the <emu-nt><a href="#prod-OptionalOperationName">OptionalOperationName</a></emu-nt>
     grammar symbol in an <emu-nt><a href="#prod-OperationRest">OperationRest</a></emu-nt>) determines the identifier of the operation.  If
     there is no such <emu-t class="regex"><a href="#prod-identifier">identifier</a></emu-t> token,
     then the operation does not have an identifier.
@@ -677,6 +677,7 @@ underscore.
 
 <pre class="grammar" id="prod-ArgumentNameKeyword">
     ArgumentNameKeyword :
+        "async"
         "attribute"
         "callback"
         "const"
@@ -1976,6 +1977,7 @@ are applicable only to regular attributes:
 
 <pre class="grammar" id="prod-AttributeNameKeyword">
     AttributeNameKeyword :
+        "async"
         "required"
 </pre>
 
@@ -2420,15 +2422,26 @@ The following extended attributes are applicable to operations:
 
 <pre class="grammar" id="prod-OperationRest">
     OperationRest :
-        OptionalIdentifier "(" ArgumentList ")" ";"
+        OptionalOperationName "(" ArgumentList ")" ";"
 </pre>
 
-<pre class="grammar" id="prod-OptionalIdentifier">
-    OptionalIdentifier :
-        identifier
+<pre class="grammar" id="prod-OptionalOperationName">
+    OptionalOperationName :
+        OperationName
         ε
 </pre>
 
+<pre class="grammar" id="prod-OperationName">
+    OperationName :
+        OperationNameKeyword
+        identifier
+</pre>
+
+<pre class="grammar" id="prod-OperationNameKeyword">
+    OperationNameKeyword :
+        "includes"
+</pre>
+
 <pre class="grammar" id="prod-ArgumentList">
     ArgumentList :
         Argument Arguments