Fixed segfault in Remove method.

runtime/vm/hash_map.h

Index: runtime/vm/hash_map.h
diff --git a/runtime/vm/hash_map.h b/runtime/vm/hash_map.h
index c50d17e87bf9e1fa3ba08d69867078ea6cd4d486..6526acaf5c6b81f5c065e78ca06a10ceea543c2f 100644
--- a/runtime/vm/hash_map.h
+++ b/runtime/vm/hash_map.h
@@ -332,6 +332,12 @@ bool BaseDirectChainedHashMap<KeyValueTrait, B, Allocator>::Remove(
 
   intptr_t current = array_[pos].next;
 
+  // If there's only the single element in the bucket and it does not match the
+  // key to be removed, just return.
+  if (current == kNil) {
+    return false;
+  }
+
   // Check the case where the second element in the bucket is the one to be
   // removed.
   if (KeyValueTrait::KeyOf(lists_[current].kv) == key) {