Eslint now allows us to use a different parser, which allows us to use
esprima-fb explicitly. This means we don't have to wait for espree to add
things like rest-param parsing. Though we do need eslint to upgrade its rules
to handle that AST.
I had hoped to enable parsing of our tests but we can't do that until we
change esprima-fb's XJS nodes to JSX.
While I was here, I also enabled the no-unused-vars rule since eslint
understands template strings. I also made the single quote enforcement
actually fail instead of just warn.
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