[dart2js] Create set of uppercase reserved symbols.
[dart2js] Create set of uppercase reserved symbols.

Change-Id: I07958be1832041a80db9c9dc6281270460d530a9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/201360
Commit-Queue: Joshua Litt <9f8daed1ab51574c9a48ffacb03e4a1adb4f880c@google.com>
Reviewed-by: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk