The dart language team will soon be changing the behavior of the
language so that an invocation of a private class members won't get
dispatched to a `noSuchMethod` method in another library; instead an
exception will be thrown.  This is a necessary prerequisite to
allowing promotion of private fields
(dart-lang/language#2020).  It also closes
an important privacy loophole in the language, by ensuring that the
user can see all the possible behaviors of a private member invocation
without having to look outside of the library.

This has an important consequence for tests using mockito, in the
scenario where a private class member is referenced from outside of
its containing class, and the containing class instance is replaced
with a mock, but the reference isn't replaced with a mock.
Previously, in this scenario, the invocation would be dispatched to
Mock.noSuchMethod invocation; with this change it will throw an
exception.

This situation arises with NavigatorObserver._navigator, which is
referenced from the NavigatorState class; if a test mocks
NavigatorObserver but not NavigatorState, it will likely fail.  It
could be argued that such tests should be rewritten to use a less
brittle mocking strategy, but there are enough examples of this
pattern in Google's internal code base that it seems likely that
it exists in code belonging to other customers too.

To avoid breaking these customers, we replace
NavigatorObserver._navigator with a static expando.  Since references
to static class members aren't affected by mocking, this will ensure
that the upcoming dart language change won't trigger any new
exceptions in code following this testing pattern.

There is no behavioral change for production code.