They previously were very sensitive to widgets like LayoutBuilder.

* Return null from VM.mainView if no view exists
* Retry in connectToServiceProtocol if a view is not yet available
* Do not explicitly call exit from the benchmarks - it will not cleanly shut down the engine

- output single-line JSON because iOS 9.x cannot output more than one line
- move the workaround for #7433 into LiveTestWidgetsFlutterBinding so that it works in all cases (e.g. animation_bench.dart on iOS)

Previously, if a StatefulWidget was marked dirty, then removed from the
build, then reinserted using the exact same widget under a widget under
a LayoutBuilder, it wouldn't rebuild.

This fixes that.

It also introduces an assert that's supposed to catch SizeObserver-like
behaviour. Rather than make this patch even bigger, I papered over two
pre-existing bugs which this assert uncovered (and fixed the other
problems it found):

   https://github.com/flutter/flutter/issues/5751
   https://github.com/flutter/flutter/issues/5749

We should fix those before 1.0 though.

This required refactoring some cases where we weren't following the rules for
the protected annotation.

Our microbenchmarks now run on real devices.