[modules] Fix bugs in assignments to exported variables.

test/mjsunit/modules-init3.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // MODULE
 
 import {check} from "modules-skip-init3.js";
 
 assertSame(undefined, w);
 assertThrows(() => x, ReferenceError);
 assertThrows(() => y, ReferenceError);
 assertThrows(() => z, ReferenceError);
 
+assertEquals(23, w = 23);
+assertThrows(() => x = 666, ReferenceError);
+assertThrows(() => y = 666, ReferenceError);
+assertThrows(() => z = 666, TypeError);
+
 export function* v() { return 40 }
 export var w = 41;
 export let x = 42;
 export class y {};
 export const z = "hello world";
 
 assertTrue(check());