VM: Fix checked mode crash (issue 13831).

runtime/vm/flow_graph_builder.cc

Index: runtime/vm/flow_graph_builder.cc
===================================================================
--- runtime/vm/flow_graph_builder.cc	(revision 29679)
+++ runtime/vm/flow_graph_builder.cc	(working copy)
@@ -27,6 +27,7 @@
 DEFINE_FLAG(bool, eliminate_type_checks, true,
             "Eliminate type checks when allowed by static type analysis.");
 DEFINE_FLAG(bool, print_ast, false, "Print abstract syntax tree.");
+DEFINE_FLAG(bool, print_scopes, false, "Print scopes of local variables.");
 DEFINE_FLAG(bool, print_flow_graph, false, "Print the IR flow graph.");
 DEFINE_FLAG(bool, print_flow_graph_optimized, false,
             "Print the IR flow graph when optimizing.");
@@ -3806,6 +3807,9 @@
     // Print the function ast before IL generation.
     AstPrinter::PrintFunctionNodes(*parsed_function());
   }
+  if (FLAG_print_scopes) {
+    AstPrinter::PrintFunctionScope(*parsed_function());
+  }
   const Function& function = parsed_function()->function();
   TargetEntryInstr* normal_entry =
       new TargetEntryInstr(AllocateBlockId(),