Add missing pieces of my last CL

runtime/observatory/tests/service/step_test.dart

Index: runtime/observatory/tests/service/step_test.dart
diff --git a/runtime/observatory/tests/service/step_test.dart b/runtime/observatory/tests/service/step_test.dart
index 47009dc490aba26ec85107030121a4faa2f76809..480f549dec40ecb04c205cdc4f6b47cd3e1d895c 100644
--- a/runtime/observatory/tests/service/step_test.dart
+++ b/runtime/observatory/tests/service/step_test.dart
@@ -32,7 +32,10 @@ Future<Isolate> stepThroughProgram(Isolate isolate) async {
   return completer.future;
 }
 
-var tests = [hasPausedAtStart, setBreakpointAtLine(LINE_A), stepThroughProgram];
+var tests = [hasPausedAtStart,
+             markDartColonLibrariesDebuggable,
+             setBreakpointAtLine(LINE_A),
+             stepThroughProgram];
 
 main(args) => runIsolateTestsSynchronous(args, tests,
     testeeConcurrent: code, pause_on_start: true, pause_on_exit: true);