Use kIsWeb instead of private _kIsBrowser in text_input.dart

[shihaohong]

Description

It seems there are multiple checks for whether the platform is web or not (checks for identical(0, 0.0). This is an attempt remove one of the places where it's defined to it comes from fewer sources.

Tests

I added the following tests:

n/a

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[fluttergithubbot]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

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[jonahwilliams]

LGTM

[Piinks]

LGTM!