[kernel] Don't output a -1 token position, it will crash observatory.

runtime/vm/kernel_binary_flowgraph.cc

Index: runtime/vm/kernel_binary_flowgraph.cc
diff --git a/runtime/vm/kernel_binary_flowgraph.cc b/runtime/vm/kernel_binary_flowgraph.cc
index 274a4654bbe6664d5add3cdc6e39fd360e602184..96b4ed696840f5bc20b37cad9d33407ec2f1c96a 100644
--- a/runtime/vm/kernel_binary_flowgraph.cc
+++ b/runtime/vm/kernel_binary_flowgraph.cc
@@ -4640,14 +4640,14 @@ TokenPosition StreamingFlowGraphBuilder::ReadPosition(bool record) {
 
 void StreamingFlowGraphBuilder::record_token_position(TokenPosition position) {
   if (record_for_script_id_ == current_script_id_ &&
-      record_token_positions_into_ != NULL) {
+      record_token_positions_into_ != NULL && position.IsReal()) {
     record_token_positions_into_->Add(position.value());
   }
 }
 
 void StreamingFlowGraphBuilder::record_yield_position(TokenPosition position) {
   if (record_for_script_id_ == current_script_id_ &&
-      record_yield_positions_into_ != NULL) {
+      record_yield_positions_into_ != NULL && position.IsReal()) {
     record_yield_positions_into_->Add(position.value());
   }
 }