tree ef67730e4c8a0e51106fba245b3eef568c78fd60
parent 8101ee0479d6628c70f63133bb8d875c447a50dc
author Jens Johansen <jensj@google.com> 1526539781 +0000
committer commit-bot@chromium.org <commit-bot@chromium.org> 1526539781 +0000

Allow to specify what dill file to initialize from

This CL updates the frontend_server to allow specifying what dill
file to initialize from.

Normally, it tries to initialize from the same dill file as is the
output. This is fine in many cases (e.g. flutter run always says it
should output to build/app.dill): The first time it compiles
everything, subsequent times (across restarts) it only recompiles
what's needed.

When running tests, however, the output is in a temporary directory,
so it doesn't work across restarts. The startup time for tests is
always rather high, because it always has to recompile everything.

This CL updates the frontend_server to allow specifying what dill
file to initialize from. Flutter can thus save the compiled test
file in a centralized location (e.g. build/test.dill) and load from
that across restarts.

Plumbing this into flutter yields the following result, when running
a single test (the one automatically created when creating a new
flutter project):

Before: ~5.5 seconds
Now, first run: ~5.5 second (unchanged)
Now, subsequent runs: ~3.4 seconds

Approximate breakdown:

Startup cost (e.g. if there's no test directory):        ~ 800 ms
Starting up the actual frontend server: ~150 ms
Reading source from old dill, loading all relevant files
and invalidating source:                                 ~ 300 ms
Compiling everything (or, rather, nothing as it's all
from the dill at this point):                            ~ 650 ms
Serializing:                                             ~ 250 ms
Copying files:                                           ~  50 ms
Running the actual test:                                 ~1100 ms

Total:                                                   ~3300 ms

`time flutter test --local-engine=host_debug --preview-dart-2`
says ~3.4 seconds...

To compare, --no-preview-dart-2 takes ~2.2 seconds,
so dart2 still has a (significantly) higher startup cost.

Moves the needle on https://github.com/flutter/flutter/issues/15767.

Change-Id: I36a2d05bc76e0193d369df42eb3b9f08416dd78f
Reviewed-on: https://dart-review.googlesource.com/51820
Reviewed-by: Kevin Millikin <kmillikin@google.com>
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>