Implement 'covariant' modifier for dart2js.

pkg/compiler/lib/src/parser/parser.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.parser;
 
 import '../common.dart';
 import '../tokens/keyword.dart' show Keyword;
 import '../tokens/precedence.dart' show PrecedenceInfo;
 import '../tokens/precedence_constants.dart'
@@ skipping 408 matching lines @@
       }
       token = parseFormalParameter(token, FormalParameterType.REQUIRED);
     } while (optional(',', token));
     listener.endFormalParameters(parameterCount, begin, token);
     return expect(')', token);
   }
 
   Token parseFormalParameter(Token token, FormalParameterType type) {
     token = parseMetadataStar(token, forParameter: true);
     listener.beginFormalParameter(token);
+
+    // Skip over `covariant` token, if the next token is an identifier or
+    // modifier.
+    // This enables the case where `covariant` is the name of the parameter:
+    //    void foo(covariant);
+    if (identical(token.stringValue, 'covariant') &&
+        token.next.isIdentifier() || isModifier(token.next)) {
+      token = token.next;
+    }
     token = parseModifiers(token);
     // TODO(ahe): Validate that there are formal parameters if void.
     token = parseReturnTypeOpt(token);
     Token thisKeyword = null;
     if (optional('this', token)) {
       thisKeyword = token;
       // TODO(ahe): Validate field initializers are only used in
       // constructors, and not for function-typed arguments.
       token = expect('.', token.next);
     }
@@ skipping 542 matching lines @@
     listener.handleModifiers(modifierCount);
     var kind = hasTypeOrModifier
         ? MessageKind.EXTRANEOUS_MODIFIER
         : MessageKind.EXTRANEOUS_MODIFIER_REPLACE;
     for (Token modifier in modifierList) {
       listener.reportError(modifier, kind, {'modifier': modifier});
     }
     return null;
   }
 
+  /// Removes the optional `covariant` token from the modifiers, if there
+  /// is no `static` in the list, and `covariant` is the first modifier.
+  Link<Token> removeOptCovariantTokenIfNotStatic(Link<Token> modifiers) {
+    if (modifiers.isEmpty ||
+        !identical(modifiers.first.stringValue, 'covariant')) {
+      return modifiers;
+    }
+    for (Token modifier in modifiers.tail) {
+      if (identical(modifier.stringValue, 'static')) {
+        return modifiers;
+      }
+    }
+    return modifiers.tail;
+  }
+
   Token parseFields(Token start, Link<Token> modifiers, Token type,
       Token getOrSet, Token name, bool isTopLevel) {
     bool hasType = type != null;
+
+    if (getOrSet == null && !isTopLevel) {
+      modifiers = removeOptCovariantTokenIfNotStatic(modifiers);
+    }
+
     Token varFinalOrConst =
         expectVarFinalOrConst(modifiers, hasType, !isTopLevel);
     bool isVar = false;
     bool hasModifier = false;
     if (varFinalOrConst != null) {
       hasModifier = true;
       isVar = optional('var', varFinalOrConst);
     }
 
     if (getOrSet != null) {
@@ skipping 1965 matching lines @@
     }
     listener.handleContinueStatement(hasTarget, continueKeyword, token);
     return expectSemicolon(token);
   }
 
   Token parseEmptyStatement(Token token) {
     listener.handleEmptyStatement(token);
     return expectSemicolon(token);
   }
 }