Change analysis server protocol to omit empty lists when optional.

pkg/analysis_server/test/analysis_notification_overrides_test.dart

 // Copyright (c) 2014, 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 test.domain.analysis.notification.overrides;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/constants.dart';
 import 'package:analysis_server/src/protocol.dart';
@@ skipping 52 matching lines @@
   void assertHasSuperElement(String search) {
     int offset = findOffset(search);
     OverriddenMember member = override.superclassMember;
     expect(member.element.location.offset, offset);
   }
 
   /**
    * Asserts that there are no overridden members from interfaces.
    */
   void assertNoInterfaceMembers() {
-    expect(override.interfaceMembers, isNull);
+    expect(override.interfaceMembers, isEmpty);
   }
 
   /**
    * Asserts that there are no overridden member from the superclass.
    */
   void assertNoSuperMember() {
     expect(override.superclassMember, isNull);
   }
 
   /**
@@ skipping 254 matching lines @@
   set fff(x) {} // in B
 }
 ''');
     return prepareOverrides().then((_) {
       assertHasOverride('fff(x) {} // in B');
       assertHasSuperElement('fff(x) {} // in A');
       assertNoInterfaceMembers();
     });
   }
 }