This reverts commit e5459942.

This is the correct metric to report for compilation time benchmarks rather
than RunTime. Rename the 'gen_snapshot' value to merely 'snapshot' for
backwards compatibility and overall simplicity.

This change simplifies Dart's benchmarking of Flutter by making it easier to
adopt --report-timings (made for Dart to use), which makes the benchmarks
much more robust.

Show appropriate error message when fonts isn't iterable

This reverts commit c0d5fd23.

* Re-enable const

* Allow adb stdout to contain the port number without failing

* Add tests that port forwarder correctly responds to known ADB output

* Update packages
* Capture JSON RPC errors that presently get swallowed