Handle Creates and Returns annotations in KernelBehaviorBuilder

pkg/compiler/lib/src/kernel/world_builder.dart

 // Copyright (c) 2017, 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.
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../common.dart';
 import '../common/backend_api.dart';
 import '../common/names.dart';
 import '../compile_time_constants.dart';
@@ skipping 89 matching lines @@
   MemberEntity lookupLibraryMember(KLibrary library, String name,
       {bool setter: false}) {
     KLibraryEnv libraryEnv = _libraryEnvs[library.libraryIndex];
     ir.Member member = libraryEnv.lookupMember(name, setter: setter);
     return member != null ? getMember(member) : null;
   }
 
   ClassEntity lookupClass(KLibrary library, String name) {
     KLibraryEnv libraryEnv = _libraryEnvs[library.libraryIndex];
     KClassEnv classEnv = libraryEnv.lookupClass(name);
-    return _getClass(classEnv.cls, classEnv);
+    if (classEnv != null) {
+      return _getClass(classEnv.cls, classEnv);
+    }
+    return null;
   }
 
   MemberEntity lookupClassMember(KClass cls, String name,
       {bool setter: false}) {
     KClassEnv classEnv = _classEnvs[cls.classIndex];
     ir.Member member = classEnv.lookupMember(name, setter: setter);
     return member != null ? getMember(member) : null;
   }
 
   ConstructorEntity lookupConstructor(KClass cls, String name) {
@@ skipping 651 matching lines @@
     }
     return builder._getLibrary(library);
   }
 
   KLibrary getLibraryForFunction(KFunction function) =>
       _getLibrary(function, builder._methodMap);
 
   KLibrary getLibraryForField(KField field) =>
       _getLibrary(field, builder._fieldMap);
 }