Function fixes

[edemaine]

The following changes are in separate commits for easier review:

• Allow suffixed expressions and statements in more places
  • Same-line body of arrow functions (fixes Coffeescript bug: postfix if #308)
  • JSX braced expressions and children
  • But not e.g. in try expression which wants postfix to be outside, and do expression which needs to treat while postfix as something completely different.
  • This is how this PR started, but then I ran into a zillion other bugs along the way...
• Forbid while/until as beginning of implicit calls
  • Matches existing behavior of for
  • Otherwise conflicts with postfix while/until notation
  • For example, x while y; got treated as a call x(while y {}) before
• Allow semicolon-separated lines in one-line =>
  • For example, => x; y, matching existing behavior for ->
  • Previously lots of weird special cases for FatArrowBody
  • Now just one case for no brace wrapping and then the usual BracedBlock
• Fix implicit return in else clause
  • Wasn't working when semicolon was inserted between then and else.
  • Improve AST so this is less brittle.
  • For example, -> if x then y else z wasn't returning z
• Fix more cases of no implicit return via semicolon
  • -> x while y; was still returning
• Wrap top-level anonymous function expressions in parens
  • Otherwise JS complains: SyntaxError: Function statements require a function name

[STRd6]

We should probably also add PostfixedExpression to no-cache rules since AssignmentExpression and ExtendedExpression are no-cache.

[edemaine]

Good point. I added PostfixedExpression and PostfixedStatement.

[STRd6]

Nice! Great work cleaning up those edge cases, lots of great fixes here!