Split MessageKind into a MessageKind key and a MessageTemplate.

pkg/compiler/lib/src/dart2jslib.dart

Index: pkg/compiler/lib/src/dart2jslib.dart
diff --git a/pkg/compiler/lib/src/dart2jslib.dart b/pkg/compiler/lib/src/dart2jslib.dart
index e6220540abf131d6ed7a34cf0113d2d467a109ed..e5c924d99e03979afae3d4b7266b4c6ea09df14e 100644
--- a/pkg/compiler/lib/src/dart2jslib.dart
+++ b/pkg/compiler/lib/src/dart2jslib.dart
@@ -64,6 +64,7 @@ import 'universe/class_set.dart';
 import 'util/characters.dart' show $_;
 import 'util/uri_extras.dart' as uri_extras show relativize;
 import 'util/util.dart';
+import 'warnings.dart';
 
 export 'helpers/helpers.dart';
 export 'resolution/resolution.dart' show TreeElements, TreeElementMapping;
@@ -77,6 +78,7 @@ export 'util/util.dart'
     show Spannable,
          CURRENT_ELEMENT_SPANNABLE,
          NO_LOCATION_SPANNABLE;
+export 'warnings.dart';
 
 part 'compiler.dart';
 part 'diagnostic_listener.dart';
@@ -84,5 +86,4 @@ part 'enqueue.dart';
 part 'resolved_visitor.dart';
 part 'script.dart';
 part 'typechecker.dart';
-part 'warnings.dart';
 part 'world.dart';