Cleanup some unused code in some of the tests

pkg/analysis_server/test/services/dependencies/reachable_source_collector_test.dart

 // Copyright (c) 2015, 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:analysis_server/src/services/dependencies/reachable_source_collector.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import '../../abstract_context.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(ReachableSourceCollectorTest);
   });
 }
 
 @reflectiveTest
 class ReachableSourceCollectorTest extends AbstractContextTest {
   Map<String, List<String>> importsFor(Source source) =>
-      new ReachableSourceCollector(source, context).collectSources();
+      new ReachableSourceCollector(source, null).collectSources();
 
   test_null_context() {
     Source lib = addSource('/lib.dart', '');
     expect(() => new ReachableSourceCollector(lib, null),
         throwsA(new isInstanceOf<ArgumentError>()));
   }
 
   @failingTest
   test_null_source() {
     // See https://github.com/dart-lang/sdk/issues/29311
     fail('The analysis.getReachableSources is not implemented.');
-    expect(() => new ReachableSourceCollector(null, context),
+    expect(() => new ReachableSourceCollector(null, null),
         throwsA(new isInstanceOf<ArgumentError>()));
   }
 
   @failingTest
   test_sources() {
     // See https://github.com/dart-lang/sdk/issues/29311
     fail('The analysis.getReachableSources is not implemented.');
     Source lib1 = addSource(
         '/lib1.dart',
         '''
@@ skipping 31 matching lines @@
         unorderedEquals([
           'dart:_internal',
           'dart:async',
           'dart:core',
           'dart:math',
           'file:///lib3.dart',
           'file:///lib4.dart'
         ]));
   }
 }