Fix crash bug when replacing a definition with a definition that has no uses.

vm/intermediate_language.cc

 // 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.
 
 #include "vm/intermediate_language.h"
 
 #include "vm/bit_vector.h"
 #include "vm/dart_entry.h"
 #include "vm/flow_graph_allocator.h"
 #include "vm/flow_graph_builder.h"
@@ skipping 294 matching lines @@
   UseVal* head = use_list();
   if (head == NULL) return;
 
   UseVal* current = head;
   while (current->next_use() != NULL) {
     current->definition_ = other;
     current = current->next_use();
   }
   current->definition_ = other;
 
-  current->next_use_ = other->use_list();
-  other->use_list()->previous_use_ = current;
+  if (other->use_list() != NULL) {
+    current->next_use_ = other->use_list();
+    other->use_list()->previous_use_ = current;
+  }
   other->set_use_list(head);
 }
 
 
 RawAbstractType* BindInstr::CompileType() const {
   ASSERT(!HasPropagatedType());
   // The compile type may be requested when building the flow graph, i.e. before
   // type propagation has occurred.
   return computation()->CompileType();
 }
@@ skipping 984 matching lines @@
   if (compiler->is_ssa()) {
     ASSERT(locs()->in(0).IsRegister());
     __ PushRegister(locs()->in(0).reg());
   }
 }
 
 
 #undef __
 
 }  // namespace dart