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

runtime/vm/kernel_binary_flowgraph.cc

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/kernel_binary_flowgraph.h"
 #include "vm/compiler.h"
 #include "vm/longjump.h"
 #include "vm/object_store.h"
 
 #if !defined(DART_PRECOMPILED_RUNTIME)
@@ skipping 4622 matching lines @@
 TokenPosition StreamingFlowGraphBuilder::ReadPosition(bool record) {
   TokenPosition position = reader_->ReadPosition();
   if (record) {
     record_token_position(position);
   }
   return position;
 }
 
 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());
   }
 }
 
 Tag StreamingFlowGraphBuilder::ReadTag(uint8_t* payload) {
   return reader_->ReadTag(payload);
 }
 
 Tag StreamingFlowGraphBuilder::PeekTag(uint8_t* payload) {
   return reader_->PeekTag(payload);
@@ skipping 3095 matching lines @@
     }
   }
 
   return Array::Handle(Array::null());
 }
 
 }  // namespace kernel
 }  // namespace dart
 
 #endif  // !defined(DART_PRECOMPILED_RUNTIME)