Add missing pieces of my last CL

runtime/observatory/tests/service/regress_28443_test.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 import 'test_helper.dart';
 import 'service_test_common.dart';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 43 matching lines @@
           reason: "Saw only $pauseEventsSeen pause events.");
       await cancelStreamSubscription(VM.kDebugStream);
       completer.complete();
     }
   });
   isolate.resume();
   return completer.future;
 }
 
 var tests = [hasPausedAtStart,
+             markDartColonLibrariesDebuggable,
              setBreakpointAtLine(LINE_B),
              resumeIsolate,
              hasStoppedAtBreakpoint,
              setBreakpointAtLine(LINE_A),
              resumeIsolate,
              hasStoppedAtBreakpoint,
              stepOut,
              stoppedAtLine(LINE_B),
              resumeIsolate];
 
 main(args) => runIsolateTestsSynchronous(args, tests,
     testeeConcurrent: test_code, pause_on_start: true, pause_on_exit: false);