This fixes a bug in Prettier where Flow generic comments get repositioned, breaking Flow's syntax. An example:

foo /*:: <bar> */ (baz);

Currently, Prettier will rewrite this (source) as:

foo(/*:: <bar> */ baz);

This PR attempts to fix this behavior. One issue that I ran into was dealing with the Flow parser's AST that it generates from the comment syntax. Code like this:

/*:: type Props = {
  foo?: ?string,
  bar: number,
}; */

gets turned into this (source):

type Props = {
  foo?: ?string,
  bar: number
};

I wasn't able to find an easy way to have Prettier reliably traverse the code backward to see if the annotation is inside a comment block. Since it's not in the AST at all (source), the options seem limited, particularly since the comment block could have begun many lines above the type definition. For this reason, this PR only ensures the correct behavior for non-inline type definitions like these for the Babylon parser. The Flow parser's behavior here remains the same as before.

• I’ve added tests to confirm my change works.
• (If changing the API or CLI) I’ve documented the changes I’ve made (in the docs/ directory)
• I’ve read the contributing guidelines.