This PR adds the Dart VM `vm:platform-const-if` pragma introduced in
https://github.com/dart-lang/sdk/commit/57a1168875 to the
`defaultTargetPlatform` property, allowing it to be computed as if it
was a constant field in non-debug AOT builds. In particular, this means
that platform-specific code executed conditionally based on this
property can be tree-shaken in release builds. Note that this PR changes
`defaultTargetPlatform` to only allow overriding via
`debugDefaultTargetPlatformOverride` in debug builds, and makes it so
that compilation throws an error if code assigns
to`debugDefaultTargetPlatformOverride` in other build modes.

Related issue: #14233

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we expect every widget to implement].
- [X] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] All existing and new tests are passing.