Adds getter/setter for slider semantics flag.

[abrush21]

Description

This change adds get and set methods for the new semantics slider flag.

Related Issues

https://github.com/abrush21/flutter/pull/new/slider-flag-2

Tests

I added the following tests:

Extended existing tests to include slider flag.

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Breaking Change

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[abrush21]

Checks expected to fail until flutter/engine#20385 lands.

[goderbauer]

Looks like the engine change has rolled into flutter. Can you rebase this onto the latest master to see if tests pass now? Thanks.

[chunhtai]

Hi @abrush21 , Do you what is the current status of this PR?

[chunhtai]

LGTM