Tests are failing: _FakeTimer doesn't have a `tick` member

[chalin]

CI tests seem to have been inadvertently disabled in #4.

> pub run test --platform vm
00:02 +0 -1: loading test/http_retry_test.dart [E]                                                                                                                                                     
  Failed to load "test/http_retry_test.dart":
  Unable to spawn isolate: file:///home/travis/.pub-cache/hosted/pub.dartlang.org/fake_async-0.1.2/lib/fake_async.dart:212:7: Error: The non-abstract class '_FakeTimer' is missing implementations for these members:
    'tick'.
  Try to either
   - provide an implementation,
   - inherit an implementation from a superclass or mixin,
   - mark the class as abstract, or
   - provide a 'noSuchMethod' implementation.

  class _FakeTimer implements Timer {
        ^^^^^^^^^^
  file:///home/travis/.pub-cache/hosted/pub.dartlang.org/fake_async-0.1.2/lib/fake_async.dart:247:61: Error: The static method has type '(dart.core::Comparable<dynamic>, dart.core::Comparable<dynamic>) → dart.core::int' that isn't of expected type '(dynamic, dynamic) → dart.core::int'.
  Change the type of the method or the context in which it is used.
  dynamic _minOf(Iterable i, [Comparator compare = Comparable.compare]) =>
                                                              ^
00:02 +0 -1: Some tests failed.  

See https://travis-ci.org/dart-lang/http_retry/builds/404507054

[chalin]

cc @kevmoo

[chalin]

Blocked by https://github.com/dart-lang/fake_async/issues/11