Add MemoryByteStore and use it in tests.

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 ab8e8ed01625a8136f9922404b35a9c78e82b15c..e8079ee98b1a21c0576f53075a08c81dd0d7ed6c 100644
--- a/pkg/analyzer/lib/src/dart/analysis/byte_store.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/byte_store.dart
@@ -30,6 +30,23 @@ abstract class ByteStore {
 }
 
 /**
+ * [ByteStore] which stores data only in memory.
+ */
+class MemoryByteStore implements ByteStore {
+  final Map<String, List<int>> _map = {};
+
+  @override
+  List<int> get(String key) {
+    return _map[key];
+  }
+
+  @override
+  void put(String key, List<int> bytes) {
+    _map[key] = bytes;
+  }
+}
+
+/**
  * A wrapper around [ByteStore] which adds an in-memory LRU cache to it.
  */
 class MemoryCachingByteStore implements ByteStore {