Add flow graph printing into a .cfg file with flag --print-flow-graph-file.

vm/isolate.h

Index: vm/isolate.h
===================================================================
--- vm/isolate.h	(revision 8145)
+++ vm/isolate.h	(working copy)
@@ -190,6 +190,14 @@
     return gc_epilogue_callbacks_;
   }
 
+  FILE* flow_graph_file() {
+    return flow_graph_file_;
+  }
+
+  void set_flow_graph_file(FILE* file) {
+    flow_graph_file_ = file;
+  }
+
  private:
   Isolate();
 
@@ -229,6 +237,7 @@
   uword spawn_data_;
   GcPrologueCallbacks gc_prologue_callbacks_;
   GcEpilogueCallbacks gc_epilogue_callbacks_;
+  FILE* flow_graph_file_;
 
   static Dart_IsolateCreateCallback create_callback_;
   static Dart_IsolateInterruptCallback interrupt_callback_;