Implement override checks for methods.

pkg/front_end/lib/src/fasta/builder/class_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.class_builder;
 
 import '../errors.dart' show internalError;
 
 import 'builder.dart'
     show
@@ skipping 36 matching lines @@
   bool get isMixinApplication => mixedInType != null;
 
   T get mixedInType;
 
   List<ConstructorReferenceBuilder> get constructorReferences => null;
 
   Map<String, Builder> get constructors;
 
   Map<String, Builder> get membersInScope => members;
 
+  LibraryBuilder get library {
+    LibraryBuilder library = parent;
+    return library.partOfLibrary ?? library;
+  }
+
   int resolveConstructors(LibraryBuilder library) {
     if (constructorReferences == null) return 0;
     Scope scope = computeInstanceScope(library.scope);
     for (ConstructorReferenceBuilder ref in constructorReferences) {
       ref.resolveIn(scope);
     }
     return constructorReferences.length;
   }
 
   Scope computeInstanceScope(Scope parent) {
@@ skipping 94 matching lines @@
             argument = argument.subst(substitutionMap);
           }
           directSubstitutionMap[variables[i]] = argument;
         }
         substitutionMap = directSubstitutionMap;
       }
     }
     return substitutionMap;
   }
 }