Fix instance variable declaration to allow covariant late final

Change Dart.g to be in sync with the language specification after
dart-lang/language#1728.

Change-Id: I7587da87ec6a0300810949a34e90af5560cbeac0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/206261
Commit-Queue: Erik Ernst <eernst@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>

Author: eernstg

tools/spec_parser/Dart.g

@@ -372,7 +372,9 @@ declaration
     |    EXTERNAL? operatorSignature
     |    STATIC (FINAL | CONST) type? staticFinalDeclarationList
     |    STATIC LATE FINAL type? initializedIdentifierList
-    |    (STATIC | COVARIANT) LATE? varOrType initializedIdentifierList
+    |    STATIC LATE? varOrType initializedIdentifierList
+    |    COVARIANT LATE FINAL type? identifierList
+    |    COVARIANT LATE? varOrType initializedIdentifierList
     |    LATE? (FINAL type? | varOrType) initializedIdentifierList
     |    redirectingFactoryConstructorSignature
     |    constantConstructorSignature (redirection | initializers)?