Simplify conversion of empty lists to null.

pkg/analysis_server/lib/src/computer/computer_overrides.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 computer.overrides;
 
+import 'package:analysis_server/src/collections.dart';
 import 'package:analysis_server/src/protocol.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart' as engine;
 
 
 /**
  * A computer for class member overrides in a Dart [CompilationUnit].
  */
 class DartUnitOverridesComputer {
   final CompilationUnit _unit;
@@ skipping 51 matching lines @@
       OverriddenMember superMember = superEngineElement != null ?
           new OverriddenMember.fromEngine(superEngineElement) :
           null;
       List<OverriddenMember> interfaceMembers = interfaceEngineElements.map(
           (engine.Element member) => new OverriddenMember.fromEngine(member)).toList();
       _overrides.add(
           new Override(
               offset,
               length,
               superclassMember: superMember,
-              interfaceMembers: interfaceMembers.isEmpty ? null : interfaceMembers));
+              interfaceMembers: nullIfEmpty(interfaceMembers)));
     }
   }
 
   static engine.Element _lookupMember(engine.ClassElement classElement,
       String name) {
     if (classElement == null) {
       return null;
     }
     engine.LibraryElement library = classElement.library;
     // method
     engine.Element member = classElement.lookUpMethod(name, library);
     if (member != null) {
       return member;
     }
     // getter
     member = classElement.lookUpGetter(name, library);
     if (member != null) {
       return member;
     }
     // setter
     member = classElement.lookUpSetter(name + '=', library);
     if (member != null) {
       return member;
     }
     // not found
     return null;
   }
 }