When building app snapshots for the SDK, train dart2js on itself.

runtime/bin/main.cc

Index: runtime/bin/main.cc
diff --git a/runtime/bin/main.cc b/runtime/bin/main.cc
index b481a67980e4721ed264860c35b2811757340efb..34bca22e3099359634427a6ce66e8bafa965d18b 100644
--- a/runtime/bin/main.cc
+++ b/runtime/bin/main.cc
@@ -1835,6 +1835,7 @@ void main(int argc, char** argv) {
 
   if (gen_snapshot_kind == kAppJIT) {
     vm_options.AddArgument("--fields_may_be_reset");
+    vm_options.AddArgument("--collect_code=false");
   }
   if ((gen_snapshot_kind == kAppAOT) || is_noopt) {
     vm_options.AddArgument("--precompilation");