Augment. Support for abstract top-level variables and static fields.

Change-Id: Idf0ce319492c405dd06364cb6feb81e000a7c741
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/506606
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>

tests/language/class_modifiers/class_modifiers_syntax_error_test.dart

@@ -43,7 +43,6 @@ final abstract class FinalAbstract {}
 // [cfe] Expected ';' after this.
 // [cfe] The modifier 'abstract' should be before the modifier 'final'.
 //             ^^^^^
-// [analyzer] COMPILE_TIME_ERROR.FINAL_NOT_INITIALIZED
 // [analyzer] SYNTACTIC_ERROR.MISSING_IDENTIFIER
 // [cfe] Expected an identifier, but got 'class'.
 base abstract class BaseAbstract {}