Fix MemoryCachingByteStore._evict after an unsuccessful get.

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 e8079ee98b1a21c0576f53075a08c81dd0d7ed6c..3f3f5dbf16ea9c3307dced03c29e3912cb392da9 100644
--- a/pkg/analyzer/lib/src/dart/analysis/byte_store.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/byte_store.dart
@@ -63,9 +63,11 @@ class MemoryCachingByteStore implements ByteStore {
     List<int> bytes = _map.remove(key);
     if (bytes == null) {
       bytes = _store.get(key);
-      _map[key] = bytes;
-      _currentSizeBytes += bytes?.length ?? 0;
-      _evict();
+      if (bytes != null) {
+        _map[key] = bytes;
+        _currentSizeBytes += bytes.length;
+        _evict();
+      }
     } else {
       _map[key] = bytes;
     }
@@ -84,6 +86,10 @@ class MemoryCachingByteStore implements ByteStore {
   void _evict() {
     while (_currentSizeBytes > _maxSizeBytes) {
       if (_map.isEmpty) {
+        // Should be impossible, since _currentSizeBytes should always match
+        // _map.  But recover anyway.
+        assert(false);
+        _currentSizeBytes = 0;
         break;
       }
       String key = _map.keys.first;