Fix source position for stack traces with optimized top function.

runtime/vm/exceptions.cc

Index: runtime/vm/exceptions.cc
===================================================================
--- runtime/vm/exceptions.cc	(revision 17448)
+++ runtime/vm/exceptions.cc	(working copy)
@@ -49,12 +49,11 @@
         InlinedFunctionsInDartFrameIterator optimized_frames(frame);
         while (true) {
           uword pc = 0;
-          func = optimized_frames.GetNextFunction(&pc);
+          func = optimized_frames.GetNextFunction(&pc, &code);
           if (func.IsNull()) {
             break;
           }
           ASSERT(pc != 0);
-          code = func.unoptimized_code();
           offset = Smi::New(pc - code.EntryPoint());
           func_list.Add(func);
           code_list.Add(code);