Fix some bugs related to the covariant keyword

pkg/analyzer/lib/src/generated/parser.dart

Index: pkg/analyzer/lib/src/generated/parser.dart
diff --git a/pkg/analyzer/lib/src/generated/parser.dart b/pkg/analyzer/lib/src/generated/parser.dart
index c90af7d69f56ef2dadcf1fe42d92585ec695d120..2a0039222a19f6f3601a3f1da83961af52797af7 100644
--- a/pkg/analyzer/lib/src/generated/parser.dart
+++ b/pkg/analyzer/lib/src/generated/parser.dart
@@ -3848,7 +3848,7 @@ class Parser {
           modifiers.constKeyword = getAndAdvance();
         }
       } else if (keyword == Keyword.COVARIANT) {
-        if (modifiers.constKeyword != null) {
+        if (modifiers.covariantKeyword != null) {
           _reportErrorForCurrentToken(
               ParserErrorCode.DUPLICATED_MODIFIER, [_currentToken.lexeme]);
           _advance();
@@ -4152,6 +4152,11 @@ class Parser {
           _tokenMatchesIdentifier(next)) {
         covariantKeyword = getAndAdvance();
       }
+      while (_matchesKeyword(Keyword.COVARIANT)) {
+        _reportErrorForCurrentToken(
+            ParserErrorCode.DUPLICATED_MODIFIER, [_currentToken.lexeme]);
+        _advance();
+      }
     }
     FinalConstVarOrType holder = parseFinalConstVarOrType(!inFunctionType,
         inFunctionType: inFunctionType);