[modules] Fix bugs in assignments to exported variables.

test/mjsunit/modules-skip-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.
 
 import {v, w, x, y, z} from "modules-init3.js";
 
 assertEquals({value: 40, done: true}, v().next());
 assertSame(undefined, w);
 assertThrows(() => x, ReferenceError);
 assertThrows(() => y, ReferenceError);
 assertThrows(() => z, ReferenceError);
 
+assertThrows(() => v = 666, TypeError);
+assertThrows(() => w = 666, TypeError);
+assertThrows(() => x = 666, TypeError);
+assertThrows(() => y = 666, TypeError);
+assertThrows(() => z = 666, TypeError);
+
 export function check() {
   assertEquals({value: 40, done: true}, v().next());
   assertEquals(41, w);
   assertEquals(42, x);
   assertEquals("y", y.name);
   assertEquals("hello world", z);
   return true;
 }