Split MessageKind into a MessageKind key and a MessageTemplate.

tests/compiler/dart2js/compiler_test.dart

Index: tests/compiler/dart2js/compiler_test.dart
diff --git a/tests/compiler/dart2js/compiler_test.dart b/tests/compiler/dart2js/compiler_test.dart
index 581489cac5686353b9710cbfc55ac362340863d5..6e16da62bae554545a7fb53062cdeda67f895ee5 100644
--- a/tests/compiler/dart2js/compiler_test.dart
+++ b/tests/compiler/dart2js/compiler_test.dart
@@ -25,7 +25,8 @@ class CallbackMockCompiler extends MockCompiler {
                      MessageKind messageKind,
                      [Map arguments = const {}]) {
     if (onWarning != null) {
-      onWarning(this, node, messageKind.message(arguments));
+      MessageTemplate template = MessageTemplate.TEMPLATES[messageKind];
+      onWarning(this, node, template.message(arguments));
     }
     super.reportWarning(node, messageKind, arguments);
   }
@@ -34,7 +35,8 @@ class CallbackMockCompiler extends MockCompiler {
                    MessageKind messageKind,
                    [Map arguments = const {}]) {
     if (onError != null) {
-      onError(this, node, messageKind.message(arguments));
+      MessageTemplate template = MessageTemplate.TEMPLATES[messageKind];
+      onError(this, node, template.message(arguments));
     }
     super.reportError(node, messageKind, arguments);
   }