Implement override checks for methods.

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 '../errors.dart' show InputError, internalError, printUnexpected;
 
 import '../export.dart' show Export;
 
 import '../loader.dart' show Loader;
 
+import '../messages.dart' show nit, warning;
+
 import '../util/relativize.dart' show relativizeUri;
 
 import 'builder.dart' show Builder, ClassBuilder, TypeBuilder;
 
 import 'scope.dart' show Scope;
 
 abstract class LibraryBuilder<T extends TypeBuilder, R> extends Builder {
   final List<Export> exporters = <Export>[];
 
   final List<InputError> compileTimeErrors = <InputError>[];
@@ skipping 28 matching lines @@
 
   void addCompileTimeError(int charOffset, Object message,
       {Uri fileUri, bool silent: false}) {
     fileUri ??= this.fileUri;
     if (!silent) {
       printUnexpected(fileUri, charOffset, message);
     }
     compileTimeErrors.add(new InputError(fileUri, charOffset, message));
   }
 
+  void addWarning(int charOffset, Object message,
+      {Uri fileUri, bool silent: false}) {
+    fileUri ??= this.fileUri;
+    if (!silent) {
+      warning(fileUri, charOffset, message);
+    }
+  }
+
+  void addNit(int charOffset, Object message,
+      {Uri fileUri, bool silent: false}) {
+    fileUri ??= this.fileUri;
+    if (!silent) {
+      nit(fileUri, charOffset, message);
+    }
+  }
+
   bool addToExportScope(String name, Builder member);
 
   void addToScope(String name, Builder member);
 
   Builder buildAmbiguousBuilder(
       String name, Builder builder, Builder other, int charOffset);
 
   int finishStaticInvocations() => 0;
 
   int finishNativeMethods() => 0;
@@ skipping 21 matching lines @@
       } else if (constructor.isFactory) {
         return constructor;
       }
     }
     throw internalError("Internal error: No constructor named"
         " '$className::$constructorName' in '$uri'.");
   }
 
   int finishTypeVariables(ClassBuilder object) => 0;
 }