Make let a contextual keyword

packages/babel-parser/src/util/identifier.js

@@ -57,7 +57,6 @@ const keywords = new Set([
   "new",
   "in",
   "this",
-  "let",
   "const",
   "class",
   "extends",
@@ -71,6 +70,8 @@ export function isKeyword(word: string): boolean {
   return keywords.has(word);
 }
 
+export const keywordRelationalOperator = /^in(stanceof)?$/;
+
 // ## Character categories
 
 // Big ugly regular expressions that matches characters in the