Fixed a small bug that happened when having a trailing comma in multiline array and object literals

lib/grammar.js

@@ -268,6 +268,8 @@
     // An **AssignList** within a block indentation.
     IndentedAssignList: [o("INDENT AssignList OUTDENT", function() {
         return $2;
+      }), o("INDENT AssignList , OUTDENT", function() {
+        return $2;
       })
     ],
     // The three flavors of function call: normal, object instantiation with `new`,
@@ -352,7 +354,7 @@
         return $1.concat([$4]);
       }), o("ArgList , INDENT Expression", function() {
         return $1.concat([$4]);
-      }), o("ArgList OUTDENT")
+      }), o("ArgList OUTDENT"), o("ArgList , OUTDENT")
     ],
     // Just simple, comma-separated, required arguments (no fancy syntax). We need
     // this to be separate from the **ArgList** for use in **Switch** blocks, where