Update co19 tests according to the Effective Dart guide

[sgrekhov]

Update co19 tests according to the Effective Dart guide

1. Use strings in Expect library parts directives. Guide link
2. Rename Expect library to expect. Guide link
3. Use lowerCamelCase for constant names in libraries and common files (at least at expect library). Guide link
4. Capitalize acronyms and abbreviations longer than two letters like words. For example isAOT -> isAot. Guide link
5. Rename abbrevations like ONE_MS to oneMillisecond. Guide link
6. Remove explicit default value null (at least in expect library, in particular in expect_async.dart). Guide link
7. dynamic_check.dart uses deprecated CastError. Do we really need it? If removing of this class doesn't introduces new test failures then remove it
8. Feel free to do more refactoring changes according to this guide. Don't try to change all 20K tests, but, at least, common libraries like expect should follow this guide