FrameTiming build start timestamp fix and add vsync start timestamp

[CareF]

Description

See flutter/flutter#62689
The original build_start time stamp is now vsync_start, and the current build time closer to what we get from timeline events.

Related Issues

fixes flutter/flutter#62689

Tests

I modify the 'FrameTiming.toString has the correct format' test in testing/dart/window_test.dart

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[iskakaushik]

when we post a trace event for VsyncSchedulingOverhead we also use fml::TimePoint::Now(). Can we unify both these and maybe post a trace event here instead?

[CareF]

Not 100% sure I understand but I made a commit moving the TaceEventAsyncComplete line. PTAL.

[CareF]

I realized that I need also to update the framework code as in flutter/flutter#62933 . How should this work? @liyuqian
Especially packages/flutter/test/scheduler/scheduler_test.dart because this test depends on the FrameTiming data length which we modify here. This will block the CI test on the framework.

[liyuqian]

I realized that I need also to update the framework code as in flutter/flutter#62933 . How should this work? @liyuqian
Especially packages/flutter/test/scheduler/scheduler_test.dart because this test depends on the FrameTiming data length which we modify here. This will block the CI test on the framework.

I'd suggest adding a new FrameTiming constructor that takes named parameter for unit tests, and document that the List<int> constructor should only be used in the engine as FramePhase could change. Here are some specific steps:

1. Introduce that named constructor in an engine PR
2. Modify the unit tests to use the named constructor in a framework PR
3. Land this PR by extending the named constructor without breaking the old unit tests
4. Update the unit test by giving the new named parameter in another framework PR (likely [engine] update for frameTiming (flutter/engine#20229) flutter#62933)

Additionally, we can make the List<int> constructor private as FrameTimng._(List<int>). The hooks.dart can still access those private constructors, but I think framework won't be able to.

[liyuqian]

Should this PR also update FrameTiming.fromTimeStamps, and make the old List<int> constructor private?

[CareF]

Should this PR also update FrameTiming.fromTimeStamps, and make the old List<int> constructor private?

I just duplicated fromTimeStamps to default constructor and will remove fromTimeStamps after everything lands. @liyuqian

And I'm guessing there may be google internal tests that's calling the old constructor, but I need confirm.

[liyuqian]

LGTM. I didn't find any reference to the FrameTiming constructors when I searched \bFrameTiming\b -file:third_party file:.dart inside the Googler internal code search tool.