Fix warning in dartium builds:

runtime/vm/debugger.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/debugger.h"
 
 #include "include/dart_api.h"
 
 #include "vm/code_generator.h"
 #include "vm/code_patcher.h"
@@ skipping 1713 matching lines @@
   return NULL;
 }
 
 
 uword Debugger::GetPatchedStubAddress(uword breakpoint_address) {
   CodeBreakpoint* bpt = GetCodeBreakpoint(breakpoint_address);
   if (bpt != NULL) {
     return bpt->saved_bytes_.target_address_;
   }
   UNREACHABLE();
-  return NULL;
+  return 0L;
 }
 
 
 // Remove and delete the source breakpoint bpt and its associated
 // code breakpoints.
 void Debugger::RemoveBreakpoint(intptr_t bp_id) {
   SourceBreakpoint* prev_bpt = NULL;
   SourceBreakpoint* curr_bpt = src_breakpoints_;
   while (curr_bpt != NULL) {
     if (curr_bpt->id() == bp_id) {
@@ skipping 90 matching lines @@
 }
 
 
 void Debugger::RegisterCodeBreakpoint(CodeBreakpoint* bpt) {
   ASSERT(bpt->next() == NULL);
   bpt->set_next(code_breakpoints_);
   code_breakpoints_ = bpt;
 }
 
 }  // namespace dart