constructor(exportName, bindingName) {
    super(bindingName);
    this.bindingName_ = bindingName;
    this.exportName_ = exportName;
  }
Replaces assignments of an identifier with an update function to update
dependent modules

a = b
a++
--a
var a = b
=>
$__export('a', a = b)
a++, $__export('a', a);
$__export('a', a++);
$__export('a', --a);
var a = b;
$__export('a', a);