Use FastaMessage instead of String. Part 1.

pkg/front_end/lib/src/fasta/builder/library_builder.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.library_builder;
 
 import '../combinator.dart' show Combinator;
 
 import '../deprecated_problems.dart' show deprecated_internalProblem;
 
 import '../export.dart' show Export;
 
 import '../loader.dart' show Loader;
 
-import '../messages.dart' show deprecated_nit, deprecated_warning;
+import '../messages.dart'
+    show Message, deprecated_nit, deprecated_warning, nit, warning;
 
 import '../util/relativize.dart' show relativizeUri;
 
 import 'builder.dart'
     show
         Builder,
         ClassBuilder,
         DynamicTypeBuilder,
         PrefixBuilder,
         Scope,
@@ skipping 44 matching lines @@
   ///
   /// If [fileUri] is null, it defaults to `this.fileUri`.
   void deprecated_addCompileTimeError(int charOffset, Object message,
       {Uri fileUri, bool silent: false, bool wasHandled: false}) {
     hasCompileTimeErrors = true;
     loader.deprecated_addCompileTimeError(
         fileUri ?? this.fileUri, charOffset, message,
         silent: silent, wasHandled: wasHandled);
   }
 
+  void addCompileTimeError(Message message, int charOffset, Uri uri,
+      {bool silent: false, bool wasHandled: false}) {
+    hasCompileTimeErrors = true;
+    loader.addCompileTimeError(message, charOffset, uri,
+        silent: silent, wasHandled: wasHandled);
+  }
+
+  void addWarning(Message message, int charOffset, Uri uri,
+      {bool silent: false}) {
+    if (!silent) {
+      warning(message, charOffset, uri);
+    }
+  }
+
+  void addNit(Message message, int charOffset, Uri uri, {bool silent: false}) {
+    if (!silent) {
+      nit(message, charOffset, uri);
+    }
+  }
+
   void deprecated_addWarning(int charOffset, Object message,
       {Uri fileUri, bool silent: false}) {
     fileUri ??= this.fileUri;
     if (!silent) {
       deprecated_warning(fileUri, charOffset, message);
     }
   }
 
   void deprecated_addNit(int charOffset, Object message,
       {Uri fileUri, bool silent: false}) {
@@ skipping 89 matching lines @@
   /// (and not a setter).
   Builder operator [](String name) {
     return scope.local[name] ??
         deprecated_internalProblem("Not found: '$name'.");
   }
 
   Builder lookup(String name, int charOffset, Uri fileUri) {
     return scope.lookup(name, charOffset, fileUri);
   }
 }