Implement override checks for methods.

pkg/front_end/lib/src/fasta/kernel/kernel_target.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.kernel_target;
 
 import 'dart:async' show Future;
 
 import 'dart:io' show File, IOSink;
 
@@ skipping 213 matching lines @@
       loader.computeLibraryScopes();
       loader.resolveTypes();
       loader.buildProgram();
       loader.checkSemantics();
       List<SourceClassBuilder> sourceClasses = collectAllSourceClasses();
       installDefaultSupertypes();
       installDefaultConstructors(sourceClasses);
       loader.resolveConstructors();
       loader.finishTypeVariables(objectClassBuilder);
       program = link(new List<Library>.from(loader.libraries));
+      loader.computeHierarchy(program);
+      loader.checkOverrides(sourceClasses);
       if (uri == null) return program;
       return await writeLinkedProgram(uri, program, isFullProgram: false);
     } on InputError catch (e) {
       return handleInputError(uri, e, isFullProgram: false);
     } catch (e, s) {
       return reportCrash(e, s, loader?.currentUriForCrashReporting);
     }
   }
 
   Future<Program> writeProgram(Uri uri,
       {bool dumpIr: false, bool verify: false}) async {
     if (loader.first == null) return null;
     if (errors.isNotEmpty) {
       return handleInputError(uri, null, isFullProgram: true);
     }
     try {
-      loader.computeHierarchy(program);
       await loader.buildBodies();
       loader.finishStaticInvocations();
       finishAllConstructors();
       loader.finishNativeMethods();
       transformMixinApplications();
       // TODO(ahe): Don't call this from two different places.
       setup_builtin_library.transformProgram(program);
       otherTransformations();
       if (dumpIr) this.dumpIr();
       if (verify) this.verify();
@@ skipping 407 matching lines @@
   }
   for (Constructor constructor in superclass.constructors) {
     if (constructor.name.name.isEmpty) {
       return constructor.function.requiredParameterCount == 0
           ? constructor
           : null;
     }
   }
   return null;
 }