investigate flaky tests

[devoncarew]

It looks like the tests introduced w/ the debugger PR are flaky on the bots:

https://travis-ci.org/flutter/devtools/jobs/474983704

We should investigate the timing issues and ensure the tests are reliable.

[devoncarew]

01:42 +6 -1: test/integration_tests/integration_test.dart: integration debugging console output [E]                                                                                                    
  Uncaught, null, 18, 0, object null
  package:webkit_inspection_protocol/src/runtime.dart 23:7  WipRuntime.evaluate
  ===== asynchronous gap ===========================
  dart:async                                                _AsyncAwaitCompleter.completeError
  package:webkit_inspection_protocol/src/runtime.dart       WipRuntime.evaluate
  ===== asynchronous gap ===========================
  dart:async                                                _asyncThenWrapperHelper
  package:webkit_inspection_protocol/src/runtime.dart       WipRuntime.evaluate
  test/integration_tests/integration.dart 154:38            BrowserTabInstance._getAppChannelObject
  test/integration_tests/integration.dart 143:22            BrowserTabInstance.getBrowserChannel
  ===== asynchronous gap ===========================
  dart:async                                                _asyncThenWrapperHelper
  test/integration_tests/integration.dart                   BrowserTabInstance.getBrowserChannel
  test/integration_tests/integration.dart 55:23             DevtoolsManager.start
  ===== asynchronous gap ===========================
  dart:async                                                _asyncThenWrapperHelper
  test/integration_tests/integration.dart                   DevtoolsManager.start
  test/integration_tests/debugger.dart 234:17               debuggingTests.<fn>
  ===== asynchronous gap ===========================
  dart:async                                                _asyncThenWrapperHelper
  test/integration_tests/debugger.dart                      debuggingTests.<fn>

[devoncarew]

Perhaps webdev serve isn't quite ready to serve assets to the browser in some cases?

chrome • Failed to load resource: the server responded with a status of 404 (Not Found)
chrome • Failed to load resource: the server responded with a status of 404 (Not Found)
chrome:error • [string Unknown error loading http://localhost:8080/lib/codemirror.js]
timeout getting the browser channel object (0:00:30.053269)

That would explain when the devtools browser object never becomes available - the web app wasn't loaded by chrome, and the initialization code didn't run.