Disallow Promise types in unions at the syntax level.

Fixes #740.

Author: Ms2ger

index.bs

@@ -5417,8 +5417,9 @@ type.
 
 <pre class="grammar" id="prod-SingleType">
     SingleType :
-        NonAnyType
+        DistinguishableType
         "any"
+        PromiseType ε
 </pre>
 
 <pre class="grammar" id="prod-UnionType">
@@ -5428,7 +5429,7 @@ type.
 
 <pre class="grammar" id="prod-UnionMemberType">
     UnionMemberType :
-        ExtendedAttributeList NonAnyType
+        ExtendedAttributeList DistinguishableType
         UnionType Null
 </pre>
 
@@ -5438,9 +5439,8 @@ type.
         ε
 </pre>
 
-<pre class="grammar" id="prod-NonAnyType">
-    NonAnyType :
-        PromiseType ε
+<pre class="grammar" id="prod-DistinguishableType">
+    DistinguishableType :
         PrimitiveType Null
         StringType Null
         identifier Null
@@ -6228,7 +6228,7 @@ and joining them with the string "<code>Or</code>".
 
 <div data-fill-with="grammar-UnionMemberTypes"></div>
 
-<div data-fill-with="grammar-NonAnyType"></div>
+<div data-fill-with="grammar-DistinguishableType"></div>
 
 
 <h4 id="idl-annotated-types">Annotated types</h4>