Address observatory analysis issues.

runtime/observatory/tests/service/async_single_step_exception_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.
 // VMOptions=--error_on_bad_type --error_on_bad_override  --verbose_debug --async_debugger
 
-import 'dart:async';
 import 'dart:developer';
-import 'package:observatory/models.dart' as M;
-import 'package:observatory/service_io.dart';
-import 'package:unittest/unittest.dart';
+
 import 'service_test_common.dart';
 import 'test_helper.dart';
 
-const LINE_A = 23;
-const LINE_B = 24;
-const LINE_C = 30;
-const LINE_D = 32;
-const LINE_E = 35;
-const LINE_F = 38;
-const LINE_G = 40;
+const LINE_A = 20;
+const LINE_B = 21;
+const LINE_C = 27;
+const LINE_D = 29;
+const LINE_E = 32;
+const LINE_F = 35;
+const LINE_G = 37;
 
 helper() async {
   print('helper'); // LINE_A.
   throw 'a'; // LINE_B.
   return null;
 }
 
 testMain() async {
   debugger();
   print('mmmmm'); // LINE_C.
@@ skipping 33 matching lines @@
   stepOver,
   hasStoppedAtBreakpoint,
   stoppedAtLine(LINE_F), // print(foo)
   stepOver,
   hasStoppedAtBreakpoint,
   stoppedAtLine(LINE_G), // print(z)
   resumeIsolate
 ];
 
 main(args) => runIsolateTests(args, tests, testeeConcurrent: testMain);