Exclude undefined from the PrimitiveType in grammar (#1140)

Exclude undefined from the PrimitiveType in grammar

Closes #1139.

Author: EdgarChen

index.bs

@@ -2253,6 +2253,10 @@ the three special floating point literal values (<emu-t>Infinity</emu-t>,
 [=optional argument/default value=],
 it is interpreted in the same way as for a [=constant=].
 
+When the <emu-t>undefined</emu-t> token is used as the
+[=optional argument/default value=],
+the value is the IDL {{undefined}} value.
+
 <div id="string-literal" algorithm="value of string literal tokens">
     Optional argument default values can also be specified using a
     <emu-t class="regex"><a href="#prod-string">string</a></emu-t>
@@ -2371,6 +2375,7 @@ Note: When defining [=method steps=], you implicitly have access to [=this=].
         "[" "]"
         "{" "}"
         "null"
+        "undefined"
 </pre>
 
 <pre class="grammar" id="prod-Operation">
@@ -5719,6 +5724,7 @@ are known as <dfn id="dfn-object-type" export>object types</dfn>.
         "FrozenArray" "&lt;" TypeWithExtendedAttributes "&gt;" Null
         "ObservableArray" "&lt;" TypeWithExtendedAttributes "&gt;" Null
         RecordType Null
+        "undefined" Null
 </pre>
 
 <div data-fill-with="grammar-ConstType"></div>
@@ -5727,7 +5733,6 @@ are known as <dfn id="dfn-object-type" export>object types</dfn>.
     PrimitiveType :
         UnsignedIntegerType
         UnrestrictedFloatType
-        "undefined"
         "boolean"
         "byte"
         "octet"