Don't crash attempting to JSON print an uninitialized Map.

runtime/observatory/tests/service/service.status

 # Copyright (c) 2014, 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.
 
 # Flaky failures
 vm_timeline_flags_test: Pass, RuntimeError # Issue 26483
 gc_test: Pass, RuntimeError # Issue 26490
 pause_on_start_and_exit_test: Pass, RuntimeError # Issue 26470
 pause_on_start_then_step_test: Pass, RuntimeError # Issue 26470
 # Reload is slow on the bots
 reload_sources_test: Pass, Slow
 
 [ ($compiler == none || $compiler == precompiler) && ($runtime == vm || $runtime == dart_precompiled) ]
 evaluate_activation_test/instance: RuntimeError # http://dartbug.com/20047
 evaluate_activation_test/scope: RuntimeError # http://dartbug.com/20047
 isolate_lifecycle_test: Pass, RuntimeError # Issue 24174
 
 # Debugger location tests are slow in debug mode.
 [ $mode == debug ]
 debugger_location_second_test: Pass, Slow
 debugger_location_test: Pass, Slow
 
-[ $mode == debug && $runtime == vm ]
-step_test: RuntimeError
-
 # Disable on simulators.
 [ $arch == simarm || $arch == simmips || $arch == simarm64 ]
 *: SkipSlow
 
 # All tests use dart:io
 [ $browser || $compiler == dart2js ]
 *: SkipByDesign
 
 # Tests with known analyzer issues
 [ $compiler == dart2analyzer ]
@@ skipping 34 matching lines @@
 # Skip all service tests because random reloads interfere.
 *: SkipByDesign # The service tests should run without being reloaded.
 
 [ $system == windows ]
 dev_fs_weird_char_test: Skip # Windows disallows question mark in paths
 dev_fs_http_put_weird_char_test: Skip # Windows disallows carriage returns in paths
 
 # Service protocol is not supported in product mode.
 [ $mode == product ]
 *: SkipByDesign