Unify reserved word checking and update error messages (#9402)

* Unify reserved word checking and update error messages

* Fix test

packages/babel-parser/src/parser/base.js

@@ -1,8 +1,6 @@
 // @flow
 
 import type { Options } from "../options";
-import { isES2015ReservedWord } from "../util/identifier";
-
 import type State from "../tokenizer/state";
 import type { PluginsMap } from "./index";
 
@@ -17,14 +15,6 @@ export default class BaseParser {
   // Initialized by Tokenizer
   state: State;
 
-  isReservedWord(word: string): boolean {
-    if (word === "await") {
-      return this.inModule;
-    } else {
-      return isES2015ReservedWord(word);
-    }
-  }
-
   hasPlugin(name: string): boolean {
     return this.plugins.has(name);
   }