Fixed segfault in Remove method.

runtime/vm/hash_map.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef RUNTIME_VM_HASH_MAP_H_
 #define RUNTIME_VM_HASH_MAP_H_
 
 #include "vm/growable_array.h"  // For Malloc, EmptyBase
 #include "vm/zone.h"
 
@@ skipping 314 matching lines @@
       lists_[next] = HashMapListElement();
       lists_[next].next = free_list_head_;
       free_list_head_ = next;
     }
     count_--;
     return true;
   }
 
   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) {
     array_[pos].next = lists_[current].next;
     lists_[current] = HashMapListElement();
     lists_[current].next = free_list_head_;
     free_list_head_ = current;
     count_--;
     return true;
   }
@@ skipping 86 matching lines @@
 
   static Key KeyOf(Pair kv) { return kv.key; }
   static Value ValueOf(Pair kv) { return kv.value; }
   static intptr_t Hashcode(Key key) { return reinterpret_cast<intptr_t>(key); }
   static bool IsKeyEqual(Pair kv, Key key) { return kv.key == key; }
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_HASH_MAP_H_