Add --print-flow-graph-filter to allow filtering debugging output.

runtime/vm/il_printer.h

Index: runtime/vm/il_printer.h
diff --git a/runtime/vm/il_printer.h b/runtime/vm/il_printer.h
index f31135764e8c449ce148b27ce46e5861c42bc4cf..c1d036f44952c5192995c8a947e39e3df8a0da3f 100644
--- a/runtime/vm/il_printer.h
+++ b/runtime/vm/il_printer.h
@@ -60,6 +60,10 @@ class FlowGraphPrinter : public ValueObject {
   // Debugging helper function.
   static void PrintICData(const ICData& ic_data);
 
+  static bool ShouldPrint(const Function& function);
+
+  static bool PassesFilter(const char* filter, const Function& function);
+
  private:
   const Function& function_;
   const GrowableArray<BlockEntryInstr*>& block_order_;