Parsing support (including incremental parsing) for 'yield' expressions.

It's their way of saying "the tokens you should parse out when you're in an expression context" (which thus contain regexps).  As opposed to the tokens you should parse when you have  "a / b".  Here, the lexical goal is the default InputElement, which only contains DivElements (like ```/```  and  ```/=```), and not regexps.