Use FastaMessage instead of String. Part 1.

pkg/front_end/lib/src/fasta/builder/library_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/library_builder.dart b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
index 6e0aa2e178056f3537156b339d6c893b942e36e0..5cc3cde1f16e2799903619763bcedc70db4f2249 100644
--- a/pkg/front_end/lib/src/fasta/builder/library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
@@ -12,7 +12,8 @@ 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;
 
@@ -77,6 +78,26 @@ abstract class LibraryBuilder<T extends TypeBuilder, R> extends Builder {
         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;