Fix issue 1857: code lookup was wrong when searching for exception handler.

runtime/vm/stack_frame.h

Index: runtime/vm/stack_frame.h
===================================================================
--- runtime/vm/stack_frame.h	(revision 4630)
+++ runtime/vm/stack_frame.h	(working copy)
@@ -132,6 +132,9 @@
   // Get function object corresponding to pc for the frame.
   RawFunction* LookupDartFunction() const;
 
+  // Get code object corresponding to pc for the frame.
+  RawCode* LookupDartCode() const;
+
   // Find exception handler pc in frame if one exists.
   bool FindExceptionHandler(uword* handler_pc) const;