Implement generalized function types.

pkg/front_end/lib/src/fasta/modifier.dart

 // Copyright (c) 2016, 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 fasta.modifier;
 
 import 'errors.dart' show internalError;
 
 enum ModifierEnum {
   Abstract,
   Const,
+  Covariant,
   External,
   Final,
   Static,
 
   // Not a real modifier.
   Var,
 }
 
 const int abstractMask = 1;
 
 const int constMask = abstractMask << 1;
 
-const int externalMask = constMask << 1;
+const int covariantMask = constMask << 1;
+
+const int externalMask = covariantMask << 1;
 
 const int finalMask = externalMask << 1;
 
 const int staticMask = finalMask << 1;
 
 const int namedMixinApplicationMask = staticMask << 1;
 
 /// Not a real modifier, and by setting it to zero, it is automatically ignored
 /// by [Modifier.validate] below.
 const int varMask = 0;
 
 const Modifier Abstract = const Modifier(ModifierEnum.Abstract, abstractMask);
 
 const Modifier Const = const Modifier(ModifierEnum.Const, constMask);
 
+const Modifier Covariant =
+    const Modifier(ModifierEnum.Covariant, covariantMask);
+
 const Modifier External = const Modifier(ModifierEnum.External, externalMask);
 
 const Modifier Final = const Modifier(ModifierEnum.Final, finalMask);
 
 const Modifier Static = const Modifier(ModifierEnum.Static, staticMask);
 
 /// Not a real modifier.
 const Modifier Var = const Modifier(ModifierEnum.Var, varMask);
 
 class Modifier {
   final ModifierEnum kind;
 
   final int mask;
 
   const Modifier(this.kind, this.mask);
 
   factory Modifier.fromString(String string) {
     if (identical('abstract', string)) return Abstract;
     if (identical('const', string)) return Const;
+    if (identical('covariant', string)) return Covariant;
     if (identical('external', string)) return External;
     if (identical('final', string)) return Final;
     if (identical('static', string)) return Static;
     if (identical('var', string)) return Var;
     return internalError("Unhandled modifier: $string");
   }
 
   toString() => "modifier(${'$kind'.substring('ModifierEnum.'.length)})";
 
   static int validate(List<Modifier> modifiers, {bool isAbstract: false}) {
-    // TODO(ahe): Implement modifier validation: ordering and uniqueness.
+    // TODO(ahe): Rename this method, validation is now taken care of by the
+    // parser.
     int result = isAbstract ? abstractMask : 0;
     if (modifiers == null) return result;
     for (Modifier modifier in modifiers) {
       result |= modifier.mask;
     }
     return result;
   }
 }