Improve flags documentation, reenable hints in dart2js.
Improve flags documentation, reenable hints in dart2js.

Now that #32111 has been fixed, we can reenable hints in dart2js. Tested on a
large consumer app and there are no unncessary hints at this time.

Change-Id: Idfe39abb319a0a8d54b4439dc74b9315d2cc1b41
Reviewed-on: https://dart-review.googlesource.com/60445
Commit-Queue: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@google.com>
Reviewed-by: Emily Fortuna <38177631bcca88ccece61f6f92cbfa56c19af864@google.com>

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