[modules] Fix bugs in assignments to exported variables.

test/mjsunit/modules-init3.js

Index: test/mjsunit/modules-init3.js
diff --git a/test/mjsunit/modules-init3.js b/test/mjsunit/modules-init3.js
index 36ff1e8ffac6d107ffd5df3c68201d45bcf02cc8..e8b46106f7e9cea0282bfbeff2ea9e0a9f637e1a 100644
--- a/test/mjsunit/modules-init3.js
+++ b/test/mjsunit/modules-init3.js
@@ -11,6 +11,11 @@ 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;