Timer from FlutterWidgetPerf causing downstream test failure

These tests pass when run alone:
`./gradlew test --tests LaunchCommandsTest`

As part of the test suite that gets run during a travis build we get this failure:
```
io.flutter.run.bazelTest.LaunchCommandsTest
✘ producesCorrectCommandLineForTestName (10.5s)
junit.framework.AssertionFailedError: Not disposed java.swing.Timer: Timer (listeners: [io.flutter.perf.FlutterWidgetPerf$$Lambda$5/1459788113@ed31ef4]) (delayed for 97ms); queue:TimerQueue (javax.swing.Timer@b53ad17)
```
This seems to be specific to travis; when run locally I cannot reproduce this failure.

Possibly related to #3610.

@jacob314

Start by running ./gradlew test --tests LaunchCommandsTest and inspect the FlutterWidgetPerf timer lifecycle alongside LaunchCommandsTest. Reproduce the Travis-only failure if possible, then verify that the full test suite no longer reports an undisposed javax.swing.Timer.

Written by the indexing model from the issue text.