Don't return negative token positions in stack traces.

runtime/vm/debugger.cc

Index: runtime/vm/debugger.cc
diff --git a/runtime/vm/debugger.cc b/runtime/vm/debugger.cc
index e485255ff6b6c4f6110c74b2f97e335ab8438946..a4afc16275873e52c3b3dac54563a4b5253b7e86 100644
--- a/runtime/vm/debugger.cc
+++ b/runtime/vm/debugger.cc
@@ -1050,7 +1050,11 @@ void ActivationFrame::PrintToJSONObject(JSONObject* jsobj,
                                         bool full) {
   const Script& script = Script::Handle(SourceScript());
   jsobj->AddProperty("type", "Frame");
-  jsobj->AddLocation(script, TokenPos());
+  TokenPosition pos = TokenPos();
+  if (pos.IsSynthetic()) {
+    pos = pos.FromSynthetic();
+  }
+  jsobj->AddLocation(script, pos);
   jsobj->AddProperty("function", function(), !full);
   jsobj->AddProperty("code", code());
   if (full) {