Improve error message for comma after rest element in arrow func

src/parser/expression.js

@@ -911,6 +911,14 @@ export default class ExpressionParser extends LValParser {
             spreadNodeStartLoc,
           ),
         );
+
+        if (this.match(tt.comma) && this.lookahead().type === tt.parenR) {
+          this.raise(
+            this.state.start,
+            "A trailing comma is not permitted after the rest element",
+          );
+        }
+
         break;
       } else {
         exprList.push(

test/fixtures/es2015/arrow-functions/params-invalid-rest-trailing-comma/actual.js

@@ -0,0 +1 @@
+(...a,) => {};

test/fixtures/es2015/arrow-functions/params-invalid-rest-trailing-comma/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "A trailing comma is not permitted after the rest element (1:5)"
+}