Adds a class `PubContext` with a fixed set of allowed values
Make it clear these values should not be changed casually
Fixed one inconsistency already: update_packages vs update_pkgs
Provide more information for verify calls
Eliminate `ctx_` prefix.
This reverts commit e13e7806e3.
Turns out that with this patch, we aren't actually catching all
errors. For example, `flutter analyze --flutter-repo --watch` didn't
report errors in `dev/devicelab/test/adb_test.dart`.
* flutter analyze --watch auto detect if in flutter repo
* move isFlutterLibrary from AnalyzeOnce into AnalyzeBase for use by AnalyzeContinuously
* pass --flutter-repo to analysis server when analyzing the flutter repository
* enhance flutter analyze --watch to summarize public members lacking documentation
