CP-2761 Add test/mock utility to define and verify exact sequence of expected requests

[evanweible-wf]

MockTransports.http.expectSequence or MockTransports.http.expectInOrder

Would probably require the addition of a public class for expectations:

class Expectation {
  Expectation.forUri(String method, Uri uri, {bool reject, Response respondWith});
  Expectation.forPattern(String method, Pattern pattern, {bool reject, Response respondWith});
  bool isMatch(String method, Uri uri);
}

MockTransports.http.expectSequence(Iterable<Expectation> expectations);

[evanweible-wf]

A more flexible approach might be to allow recording HTTP requests & WebSocket connections for a finite period of time. See #240