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

There are different parsing contexts where you have different "goals" for what your lexer produces; in this context, you try to look for a RegExp when you lex. There's a similar one for template expressions.

https://people.mozilla.org/~jorendorff/es6-draft.html#sec-ecmascript-language-lexical-grammar
>The source text of an ECMAScript script is first converted into a sequence of input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of code units as the next input element.

>There are several situations where the identification of lexical input elements is sensitive to the syntactic grammar context that is consuming the input elements. This requires multiple goal symbols for the lexical grammar. The ***InputElementDiv*** goal symbol is the default goal symbol and is used in those syntactic grammar contexts where a leading division (/) or division-assignment (/=) operator is permitted. The ***InputElementRegExp*** goal symbol is used in all syntactic grammar contexts where a ***RegularExpressionLiteral*** is permitted. The ***InputElementTemplateTail*** goal is used in syntactic grammar contexts where a ***TemplateLiteral*** logically continues after a substitution element.

In our parser, we just rescan. I think it's partially because it helps with error handling capabilities, but it's also just cheap because we only rescan with single-character tokens anyhow.