Make Observatory analyzer clean

runtime/observatory/tests/service/async_scope_test.dart

 // Copyright (c) 2015, 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=--compile_all --error_on_bad_type --error_on_bad_override
 
-import 'dart:async';
 import 'dart:developer';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 
 foo() {}
 
 doAsync(param1) async {
   var local1 = param1 + 1;
-  foo(); // Line 16
-  await null;
+  foo(); // Line 15
+  await local1;
 }
 
 doAsyncStar(param2) async* {
   var local2 = param2 + 1;
-  foo(); // Line 22
-  yield null;
+  foo(); // Line 21
+  yield local2;
 }
 
 testeeDo() {
   debugger();
 
   doAsync(1).then((_) {
     doAsyncStar(1).listen((_) {});
   });
 }
 
@@ skipping 13 matching lines @@
   expect(stack.type, equals('Stack'));
   expect(stack['frames'].length, greaterThanOrEqualTo(1));
   Frame frame = stack['frames'][0];
   var vars = frame.variables.map((v) => v['name']).join(' ');
   expect(vars, equals('param2 local2')); // no :async_op et al
 }
 
 
 var tests = [
   hasStoppedAtBreakpoint, // debugger()
-  setBreakpointAtLine(16),
-  setBreakpointAtLine(22),
+  setBreakpointAtLine(15),
+  setBreakpointAtLine(21),
   resumeIsolate,
 
   hasStoppedAtBreakpoint,
-  stoppedAtLine(16),
+  stoppedAtLine(15),
   checkAsyncVarDescriptors,
   resumeIsolate,
 
   hasStoppedAtBreakpoint,
-  stoppedAtLine(22),
+  stoppedAtLine(21),
   checkAsyncStarVarDescriptors,
   resumeIsolate,
 ];
 
 main(args) => runIsolateTests(args, tests, testeeConcurrent: testeeDo);