Add NullByteStore, that does not store any data.

pkg/analyzer/lib/src/dart/analysis/byte_store.dart

Index: pkg/analyzer/lib/src/dart/analysis/byte_store.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/byte_store.dart b/pkg/analyzer/lib/src/dart/analysis/byte_store.dart
index 3f3f5dbf16ea9c3307dced03c29e3912cb392da9..2f151f9678738767cb7536ada71a3078a88e9a3a 100644
--- a/pkg/analyzer/lib/src/dart/analysis/byte_store.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/byte_store.dart
@@ -98,3 +98,14 @@ class MemoryCachingByteStore implements ByteStore {
     }
   }
 }
+
+/**
+ * [ByteStore] which does not store any data.
+ */
+class NullByteStore implements ByteStore {
+  @override
+  List<int> get(String key) => null;
+
+  @override
+  void put(String key, List<int> bytes) {}
+}