Don't return negative token positions in stack traces.

runtime/observatory/tests/service/service_test_common.dart

Index: runtime/observatory/tests/service/service_test_common.dart
diff --git a/runtime/observatory/tests/service/service_test_common.dart b/runtime/observatory/tests/service/service_test_common.dart
index 4a42290543cc00e9d231d212d3e0e76de7dc30b0..3373fa4434b1ae4a5007d96648a099456092a09d 100644
--- a/runtime/observatory/tests/service/service_test_common.dart
+++ b/runtime/observatory/tests/service/service_test_common.dart
@@ -233,6 +233,11 @@ Future<Isolate> stepOver(Isolate isolate) async {
   return hasStoppedAtBreakpoint(isolate);
 }
 
+Future<Isolate> stepInto(Isolate isolate) async {
+  await isolate.stepInto();
+  return hasStoppedAtBreakpoint(isolate);
+}
+
 Future<Class> getClassFromRootLib(Isolate isolate, String className) async {
   Library rootLib = await isolate.rootLibrary.load();
   for (var i = 0; i < rootLib.classes.length; i++) {