Add NullByteStore, that does not store any data.

pkg/analyzer/test/src/dart/analysis/byte_store_test.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.
 
 import 'package:analyzer/src/dart/analysis/byte_store.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:typed_mock/typed_mock.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(MemoryCachingByteStoreTest);
+    defineReflectiveTests(NullByteStoreTest);
   });
 }
 
+List<int> _b(int length) {

[Brian Wilkerson, 2017/01/26 14:51:18]

This is only used in one place. Please consider inlining it, or at the very
least use a name that has some semantic value.

+  return new List<int>(length);
+}
+
 @reflectiveTest
 class MemoryCachingByteStoreTest {
   test_get_notFound_evict() {
     var store = new _TestByteStore();
     var cachingStore = new MemoryCachingByteStore(store, 100);
 
     // Request '1'.  Nothing found.
     cachingStore.get('1');
 
     // Add enough data to the store to force an eviction.
@@ skipping 66 matching lines @@
     expect(cachingStore.get('1'), hasLength(10));
     expect(cachingStore.get('2'), hasLength(80));
 
     // 10 + 80 + 30 > 100
     // So, '1' and '2' are evicted.
     cachingStore.put('3', _b(30));
     expect(cachingStore.get('1'), isNull);
     expect(cachingStore.get('2'), isNull);
     expect(cachingStore.get('3'), hasLength(30));
   }
+}
 
-  static List<int> _b(int length) {
-    return new List<int>(length);
+@reflectiveTest
+class NullByteStoreTest {
+  test_get() {
+    var store = new NullByteStore();
+
+    expect(store.get('1'), isNull);
+
+    store.put('1', _b(10));
+    expect(store.get('1'), isNull);
   }
 }
 
 class _TestByteStore extends TypedMock implements ByteStore {}