Turn out-of-module writes to module properties into calls to setters

[rictic]

Related to Polymer/polymer#4754

[rictic]

e.g. in polymer/test/unit/property-effects.html we've got code like:

    Polymer.sanitizeDOMValue = sinon.spy(function(value) {
      return (value && value.toString().indexOf('javascript') >= 0) ? 'notallowed!' : value;
    });

currently that becomes:

import { sanitizeDOMValue as $sanitizeDOMValue } from '../../lib/utils/settings.js';

// ...

    $sanitizeDOMValue = sinon.spy(function(value) {
      return (value && value.toString().indexOf('javascript') >= 0) ? 'notallowed!' : value;
    });

which is illegal (as you can't write to a module export). Instead it should become:

import { sanitizeDOMValue as $sanitizeDOMValue, setSanitizeDOMValue as $setSanitizeDOMValue } from '../../lib/utils/settings.js';

// ...

    $setSanitizeDOMValue(sinon.spy(function(value) {
      return (value && value.toString().indexOf('javascript') >= 0) ? 'notallowed!' : value;
    }));

[rictic]

Fixed!