Fix all Observatory warnings/errors.

runtime/observatory/test/caching_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 --checked
 
 // If caching is working properly, the coverage data will go into the same
 // Script object from which we requested coverage data, instead of a new
 // Script object.
 
 library caching_test;
 
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
-import 'dart:async';
 
 script() {
   print("This executed");
 }
 
 hasSomeCoverageData(Script script) {
   for (var line in script.lines) {
     if (line.hits != null) return true;
   }
   return false;
 }
 
 var tests = [
 (Isolate isolate) async {
   Library lib = await isolate.rootLib.load();
   Script script = await lib.scripts.single.load();
   expect(hasSomeCoverageData(script), isFalse);
   Script script2 = await script.refreshCoverage();
   expect(identical(script, script2), isTrue);
   expect(hasSomeCoverageData(script), isTrue);
 },
 
 ];
 
 main(args) => runIsolateTests(args, tests, testeeBefore: script);