Fix erroneous pattern parsing inside ternary expressions

[IwanKaramazow]

Fixes #2155

The problem:

true ? (Update({...a, b: 1}), None) : x;

We "over"-parse spread operators inside patterns for better errors.
Because of the parens, semicolon : and ... now being a valid codepath, the lexer triggers an injection of an ES6_FUN token. This is clearly not an es6 style function expression.
As soon as a ... is reached with a closing }, we know that we definitely don't have an es6 function expression. By exiting the lexer's es6-procedure at this stage, the parser continues it's normal behaviour and parses the correct expression.