Use FastaMessage instead of String. Part 1.

pkg/front_end/lib/src/fasta/messages.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.messages;
 
 import 'package:kernel/ast.dart' show Library, Location, Program, TreeNode;
 
 import 'util/relativize.dart' show relativizeUri;
 
 import 'compiler_context.dart' show CompilerContext;
 
 import 'deprecated_problems.dart' show deprecated_InputError;
 
 import 'colors.dart' show cyan, magenta;
 
+import 'fasta_codes.dart' show Message;
+
+export 'fasta_codes.dart';
+
 const bool hideWarnings = false;
 
 bool get errorsAreFatal => CompilerContext.current.options.errorsAreFatal;
 
 bool get nitsAreFatal => CompilerContext.current.options.nitsAreFatal;
 
 bool get warningsAreFatal => CompilerContext.current.options.warningsAreFatal;
 
 bool get isVerbose => CompilerContext.current.options.verbose;
 
 bool get hideNits => !isVerbose;
 
+void warning(Message message, int charOffset, Uri uri) {
+  if (hideWarnings) return;
+  print(deprecated_format(
+      uri, charOffset, colorWarning("Warning: ${message.message}")));
+  if (warningsAreFatal) {
+    if (isVerbose) print(StackTrace.current);
+    throw new deprecated_InputError(
+        uri, charOffset, "Compilation aborted due to fatal warnings.");
+  }
+}
+
+void nit(Message message, int charOffset, Uri uri) {
+  if (hideNits) return;
+  print(
+      deprecated_format(uri, charOffset, colorNit("Nit: ${message.message}")));
+  if (nitsAreFatal) {
+    if (isVerbose) print(StackTrace.current);
+    throw new deprecated_InputError(
+        uri, charOffset, "Compilation aborted due to fatal nits.");
+  }
+}
+
 void deprecated_warning(Uri uri, int charOffset, String message) {
   if (hideWarnings) return;
   print(deprecated_format(uri, charOffset, colorWarning("Warning: $message")));
   if (warningsAreFatal) {
     if (isVerbose) print(StackTrace.current);
     throw new deprecated_InputError(
         uri, charOffset, "Compilation aborted due to fatal warnings.");
   }
 }
 
@@ skipping 69 matching lines @@
       program.libraries.clear();
       parent.parent = null;
       return result;
     } else {
       return null;
     }
   } else {
     return node.location;
   }
 }