Part of https://github.com/flutter/flutter/issues/160043, makes it
easier to add https://github.com/flutter/flutter/pull/160131.
This PR has no functional changes to any of the code, but does refactor
both the code and tests:
- Makes a number of always non-null but not migrated to non-null
properties, well, not-null
- Creates two concrete methods (`update` and `compare` versus a
positional nullable boolean)
- Uses type signatures instead of `String?` to explain the possible
results of the methods
- Renames the mysterious `shellPath` variable to `flutterTesterBinPath`
- Expands and rewrites internally-facing doc comments
- Moves `WebRenderer` environment variable setting to
`flutter_web_platform.dart`
- Makes the tests have less duplication, and check for update/compare
cases
After this PR, I can use it in the non-web branch of the Flutter tool
without any hacks or TODOS :)
/cc @eyebrowsoffire (trivial web refactoring), @camsim99 (changes being
made to tool).
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