[web_ui] add missing dispose handler for MethodCalls to flutter/platform_view

[tauu]

This PR adds the missing handler for the 'dispose' MethodCall to the PlatformViewChannel ( flutter/platform_view ). This MethodCall is already invoked by _HtmlElementViewController in https://github.com/flutter/flutter/blob/2c94f2b4d9247965a32a873878cabfe6d2fb7a93/packages/flutter/lib/src/widgets/platform_view.dart#L412 and yields a MissingPluginException Error without the corresponding implementation.

Fixes #12226

[yjbanov]

/cc @hterkelsen

[harryterkelsen]

Please format this comment as a full sentence with punctuation, i.e., // Remove ... DOM.

[harryterkelsen]

Ping

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[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[harryterkelsen]

LGTM. Looks like the tests are failling right now. Please wait to land this until the tests are passing again, as it seems unrelated to your change.

[tauu]

OK, thx! :-) Sorry for the delay. Migrating from the flutter_web fork, as well as integrating a custom flutter engine build into the build system of my team took longer than expected. (Still can't figure out how to build only the web_sdk part of the engine. :-/ )

[cbracken]

Looks like compiler error on the latest run:

[ +383 ms] Dart2Js finished with:
           packages/flutter/src/semantics/semantics.dart:2060:7:
           Error: No named parameter with the name 'maxValueLength'.
                 maxValueLength: data.maxValueLength ?? -1,
                 ^^^^^^^^^^^^^^
           Error: Compilation failed.

If this is unrelated to your PR, can you try rebasing against head? It's possible this was an error we fixed in the meantime.

Thanks for your PR. We'll keep an eye out for when it's passing.

[cbracken]

Looks like compiler error on the latest run:

[ +383 ms] Dart2Js finished with:
           packages/flutter/src/semantics/semantics.dart:2060:7:
           Error: No named parameter with the name 'maxValueLength'.
                 maxValueLength: data.maxValueLength ?? -1,
                 ^^^^^^^^^^^^^^
           Error: Compilation failed.

Thanks for your PR. We'll keep an eye out for when it's passing.

[tauu]

The compiler error was unrelated to the PR and rebasing fixed the failing tests.