Fix and test for MemoryCachingByteStore.

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 681ee25831abdf99a900f7f982ee529accd3f98a..ab8e8ed01625a8136f9922404b35a9c78e82b15c 100644
--- a/pkg/analyzer/lib/src/dart/analysis/byte_store.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/byte_store.dart
@@ -43,12 +43,14 @@ class MemoryCachingByteStore implements ByteStore {
 
   @override
   List<int> get(String key) {
-    List<int> bytes = _map[key];
+    List<int> bytes = _map.remove(key);
     if (bytes == null) {
       bytes = _store.get(key);
       _map[key] = bytes;
       _currentSizeBytes += bytes?.length ?? 0;
       _evict();
+    } else {
+      _map[key] = bytes;
     }
     return bytes;
   }