Fail speculative parsing of arrow function expressions whenever it has a
parameter with an initialiser that is missing '='. Ordinarily this is
allowed for better error recovery in the language service, but for
speculative parsing, the errors can compound. When the initialiser is an
error, and when the '=>' is missing (which is also allowed), what is
putatively an arrow function may actually be something else.
For example, `(a / 8) + function ()
{ }` is currently parsed as if someone had intended to write
`(a = /8)+function()/) => { }` but they forgot the `=` of the
initialiser, the `=>` of the lambda, forgot to close the regular
expression, and mistakenly inserted a newline right after the regular
expression.
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