[analyzer] Introduce WarningCode for non-spec warnings.
Rename StaticWarningCode to WarningCode, leaving the spec ones as StaticWarningCode.
In this CL I do not introduce a new "type" for WarningCode, it is still "STATIC_WARNING". Later, we can make a new type, "WARNING", taking care for pkg/test_runner and DAS clients like IntelliJ.
Bug: https://github.com/dart-lang/sdk/issues/50796
Change-Id: Ife856b2beb7e9b6bf8203f365634f49846d03bf1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/278997
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Sam Rawlins
305358e8ce