Switch our pubspec.yamls to using SDK sources so that we can have consistent
source types when we depend on these packages from external packages using SDK
sources.
...by adding tests to our examples that don't import flutter_test, which
pins the relevant dependencies.
Also, provide more information when complaining about leaked transient
callbacks in tests.
Also, make tests display full information when they have an exception,
by bypassing the throttling we have for Android logging in tests.
Also, make the word wrapping not wrap stack traces if they happen to
be included in exception output.
Also, fix a leaked transient callback in the checkbox code.