Fix regression with let (#9477)

* Fix corner cases with let

* Handle generators correctly

* Fix flow plugin

* Fix typescript plugin

packages/babel-parser/src/plugins/flow.js

@@ -1566,7 +1566,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
     }
 
     // interfaces
-    parseStatement(declaration: boolean, topLevel?: boolean): N.Statement {
+    parseStatement(context: ?string, topLevel?: boolean): N.Statement {
       // strict mode handling of `interface` since it's a reserved word
       if (
         this.state.strict &&
@@ -1577,7 +1577,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         this.next();
         return this.flowParseInterface(node);
       } else {
-        const stmt = super.parseStatement(declaration, topLevel);
+        const stmt = super.parseStatement(context, topLevel);
         // We will parse a flow pragma in any comment before the first statement.
         if (this.flowPragma === undefined && !this.isValidDirective(stmt)) {
           this.flowPragma = null;

packages/babel-parser/src/plugins/typescript.js

@@ -1699,10 +1699,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       return super.parseExportDefaultExpression();
     }
 
-    parseStatementContent(
-      declaration: boolean,
-      topLevel: ?boolean,
-    ): N.Statement {
+    parseStatementContent(context: ?string, topLevel: ?boolean): N.Statement {
       if (this.state.type === tt._const) {
         const ahead = this.lookahead();
         if (ahead.type === tt.name && ahead.value === "enum") {
@@ -1712,7 +1709,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
           return this.tsParseEnumDeclaration(node, /* isConst */ true);
         }
       }
-      return super.parseStatementContent(declaration, topLevel);
+      return super.parseStatementContent(context, topLevel);
     }
 
     parseAccessModifier(): ?N.Accessibility {