analyzer: Convert Flutter (utilities) into a static class.

This was a utility class with absolutely zero state. It almost protected
against unnecessary instantiation by providing a static `instance`
member, but the constructor was still public. Also, the instance was
passed around various objects as an instance field, even allowing for
the static instance to be nullable in a few cases. All of this can be
prevented by making the class `abstract final`, with all of its members
static.

(An argument can be made that all of the code in here could be made
top-level functions, but I'll leave that for a later refactoring, if it
is desired. Also the functions could be made into extension methods, but
many of them rely on promotion of a parameter, so until we get `this`
promotion in extensions, I won't advocate for that change.)

The main reason I was looking at this was to get Flutter out of `_AbstractCorrectionProducer`, so that we don't have to drag the flutter utils into analyzer_plugin (we still can if we want to, but we won't _have_ to).

I also modernized the code, following Effective Dart:

* https://dart.dev/effective-dart/documentation#do-start-doc-comments-with-a-single-sentence-summary
* https://dart.dev/effective-dart/documentation#do-separate-the-first-sentence-of-a-doc-comment-into-its-own-paragraph
* https://dart.dev/effective-dart/documentation#prefer-starting-function-or-method-comments-with-third-person-verbs
* https://dart.dev/effective-dart/documentation#prefer-starting-a-boolean-variable-or-property-comment-with-whether-followed-by-a-noun-or-gerund-phrase

Change-Id: I0d1896ae2d45a31e00f8a824e92d173469cc7eea
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/340301
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>