[messages] Clean up SharedCode enum.

The enumerated values in the code-generated `SharedCode` enum are now named after the camelCase analyzer error code, and sorted accordingly. This makes the enum look a lot more natural and Dart-like. (Previously, the enumerated values were capitalized and the sort order wasn't obvious). The corresponding `sharedAnalyzerCodes` list (which is code-generated to match the order of the `SharedCode` enum) is suitably adjusted to reflect the new sort order, so there is no functional change. Addresses code review comment https://dart-review.googlesource.com/c/sdk/+/455466/comment/07c4632c_1c93e509/. Change-Id: I6a6a696469ed5c8a814e3531b666bb358979cde0 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/455761 Reviewed-by: Konstantin Shcheglov <scheglov@google.com> Commit-Queue: Paul Berry <paulberry@google.com>

Author: stereotype441