Fix evaluator bug when expression starts with let or const

[danddanddand]

Fix bug where expressions that start with let/const were not evaluating properly.

This is sample that wasn't evaluating properly:

<ul x-data="{ letters: ['a','b','c'] }">
  <template x-for="letter in letters">
    <li x-text="letter"></li>
  </template>
</ul>

Reported here #2223

[calebporzio]

Oh good catch - thanks!