Fix flaky standalone/io/TestRunnerTest, a unit test of the testing framework.
Fix flaky standalone/io/TestRunnerTest, a unit test of the testing framework.

BUG=dart:1947
TEST=standalone/io/TestRunnerTest

Review URL: https://chromiumcodereview.appspot.com//10078011

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@6734 260f80e4-7a28-3924-810f-c04153c831b5

dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk