[Spec] <declaration> production seems to be missing some types of modifiers

This code can be compiled by the Dart compiler, but it can't be parse with the given rules in the Spec:
```
abstract class StreamController {
final int i1 = 0;
abstract int i2; // Can't parse
abstract final int i3; // Can't parse
}
```
It fails to parse the declaration for i2 and i3 because of the modifier "abstract".

From the Spec, the rule for [declaration](https://github.com/dart-lang/language/blob/812f939ec0130da119ad2cbe5f9a4121e31b0831/specification/dartLangSpec.tex#L2423) allows for a number of cases, but does not allow for a declaration that begins with `abstract`.

Note, for a class member declaration, the parser itself seems to recognize [three groups of modifiers](https://github.com/dart-lang/sdk/blob/81584f5d031fc12049f954b8510de7c36f1e56ea/pkg/_fe_analyzer_shared/lib/src/parser/parser_impl.dart#L3918-L3977) followed by a [list of modifiers](), roughly as ('external'|'augment'|'abstract')? ('static'|'covariant')? ('final'|'var'|'const'|('late' 'final'?)? [classMemberModifiers](https://github.com/dart-lang/sdk/blob/81584f5d031fc12049f954b8510de7c36f1e56ea/pkg/_fe_analyzer_shared/lib/src/parser/parser_impl.dart#L3965)?; [classMemberModifiers](https://github.com/dart-lang/sdk/blob/81584f5d031fc12049f954b8510de7c36f1e56ea/pkg/_fe_analyzer_shared/lib/src/parser/modifier_context.dart#L96) : [modifiers](https://github.com/dart-lang/sdk/blob/81584f5d031fc12049f954b8510de7c36f1e56ea/pkg/_fe_analyzer_shared/lib/src/parser/modifier_context.dart#L97); [modifiers](https://github.com/dart-lang/sdk/blob/81584f5d031fc12049f954b8510de7c36f1e56ea/pkg/_fe_analyzer_shared/lib/src/parser/modifier_context.dart#L245) : (abstract|augment|const|covariant|external|final|late|required|static|var)*. Anyway, the CFG in the Spec needs alts to allow `abstract` members of a class

Start with the declaration production in specification/dartLangSpec.tex around line 2423 and compare it with the abstract member examples in the issue. Review the referenced parser modifier handling in pkg/_fe_analyzer_shared/lib/src/parser/parser_impl.dart and modifier_context.dart. Done means the specification grammar accepts the shown abstract fields and the relevant specification checks pass.

Written by the indexing model from the issue text.