Add missing pieces of my last CL

runtime/observatory/tests/service/step_test.dart

 import 'dart:async';
 import 'test_helper.dart';
 import 'service_test_common.dart';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 
 const int LINE_A = 10;
 
 code() {
   var x = {}; // LINE_A
@@ skipping 14 matching lines @@
       expect(pauseEventsSeen > 20, true,
           reason: "Saw only $pauseEventsSeen pause events.");
       await cancelStreamSubscription(VM.kDebugStream);
       completer.complete();
     }
   });
   isolate.resume();
   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);