Implement generalized function types.

pkg/compiler/lib/src/util/util.dart

Index: pkg/compiler/lib/src/util/util.dart
diff --git a/pkg/compiler/lib/src/util/util.dart b/pkg/compiler/lib/src/util/util.dart
index 457377800edd33993e4933dba62becc6fce7f83c..d96c1e350c20ed326f75b3c06dc4f9ad447f8f0a 100644
--- a/pkg/compiler/lib/src/util/util.dart
+++ b/pkg/compiler/lib/src/util/util.dart
@@ -191,7 +191,8 @@ String modifiersToString(
     bool isVar: false,
     bool isConst: false,
     bool isFactory: false,
-    bool isExternal: false}) {
+    bool isExternal: false,
+    bool isCovariant: false}) {
   LinkBuilder<String> builder = new LinkBuilder<String>();
   if (isStatic) builder.addLast('static');
   if (isAbstract) builder.addLast('abstract');
@@ -200,6 +201,7 @@ String modifiersToString(
   if (isConst) builder.addLast('const');
   if (isFactory) builder.addLast('factory');
   if (isExternal) builder.addLast('external');
+  if (isCovariant) builder.addLast('covariant');
   StringBuffer buffer = new StringBuffer();
   builder.toLink().printOn(buffer, ', ');
   return buffer.toString();