Add DISALLOW_COPY_AND_ASSIGN to subclasses of ValueObject and make ValueObject copyable.

runtime/vm/hash_map.h

Index: runtime/vm/hash_map.h
===================================================================
--- runtime/vm/hash_map.h	(revision 18957)
+++ runtime/vm/hash_map.h	(working copy)
@@ -50,6 +50,10 @@
   // with a given hash.  Colliding elements are stored in linked lists.
   HashMapListElement* lists_;  // The linked lists containing hash collisions.
   intptr_t free_list_head_;  // Unused elements in lists_ are on the free list.
+
+  // Disallow operator=. Use public copy constructor to copy instead.
+  DirectChainedHashMap<KeyValueTrait>& operator=(
+      const DirectChainedHashMap& other);
 };