Implement override checks for methods.

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

Index: pkg/front_end/lib/src/fasta/builder/class_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/class_builder.dart b/pkg/front_end/lib/src/fasta/builder/class_builder.dart
index b85c794fa14cb4abde5781ed1eecfbb28fc57ea8..991c2367d7eaafb31fb03ee4667288a12478d115 100644
--- a/pkg/front_end/lib/src/fasta/builder/class_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/class_builder.dart
@@ -54,6 +54,11 @@ abstract class ClassBuilder<T extends TypeBuilder, R>
 
   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);