Add an API function to get a debugger stack trace from an error handle.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 1a2e1d20b5957ae7c775ace520d48a6b86895c94..9961e8651847ca78c7fa91dbe186d5b7c7261f86 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -6016,6 +6016,13 @@ class Stacktrace : public Instance {
   RawString* FullStacktrace() const;
   const char* ToCStringInternal(intptr_t* frame_index) const;
 
+  intptr_t VisibleLength() const;
+  bool VisibleFrameInfoAt(intptr_t frame_index,
+                          String* internal_function_name,
+                          String* internal_script_url,
+                          intptr_t* internal_line_number,
+                          intptr_t* internal_col_num) const;
+
  private:
   void set_code_array(const Array& code_array) const;
   void set_pc_offset_array(const Array& pc_offset_array) const;