Clean up error handling.

dart/sdk/lib/_internal/compiler/implementation/typechecker.dart

Index: dart/sdk/lib/_internal/compiler/implementation/typechecker.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/typechecker.dart b/dart/sdk/lib/_internal/compiler/implementation/typechecker.dart
index 5249a6e768f1605a59f42c4c22383764282bac2d..ce2fdb1e16cdd7a6e45878d2802e4f5a8fb71917 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/typechecker.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/typechecker.dart
@@ -237,7 +237,7 @@ class TypeCheckerVisitor extends Visitor<DartType> {
    */
   bool checkAssignable(Node node, DartType from, DartType to) {
     if (!types.isAssignable(from, to)) {
-      reportTypeWarning(node, MessageKind.NOT_ASSIGNABLE,
+      reportTypeWarning(node, MessageKind.NOT_ASSIGNABLE.warning,
                         {'fromType': from, 'toType': to});
       return false;
     }
@@ -413,7 +413,7 @@ class TypeCheckerVisitor extends Visitor<DartType> {
             {'className': type.name, 'memberName': name});
         break;
       case MemberKind.GETTER:
-        reportTypeWarning(node, MessageKind.MEMBER_NOT_FOUND,
+        reportTypeWarning(node, MessageKind.MEMBER_NOT_FOUND.warning,
             {'className': type.name, 'memberName': name});
         break;
       case MemberKind.SETTER: