Fix debugger stepping

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 305 matching lines @@
   if (pc_desc_.IsNull()) {
     pc_desc_ = code().pc_descriptors();
     ASSERT(!pc_desc_.IsNull());
   }
 }
 
 
 // Compute token_pos_ and pc_desc_index_.
 intptr_t ActivationFrame::TokenPos() {
   if (token_pos_ < 0) {
+    token_pos_ = Scanner::kNoSourcePos;
     GetPcDescriptors();
     for (intptr_t i = 0; i < pc_desc_.Length(); i++) {
       if (pc_desc_.PC(i) == pc_) {
         pc_desc_index_ = i;
         token_pos_ = pc_desc_.TokenPos(i);
         break;
       }
     }
   }
   return token_pos_;
@@ skipping 1921 matching lines @@
 }
 
 
 void Debugger::RegisterCodeBreakpoint(CodeBreakpoint* bpt) {
   ASSERT(bpt->next() == NULL);
   bpt->set_next(code_breakpoints_);
   code_breakpoints_ = bpt;
 }
 
 }  // namespace dart