[webview_flutter] Expose loadData function for webviews

[GeertJohan]

Description

Expose loadData on the webview controller. This allows developers to load html directly into the webview.

This is a draft PR. I'm using this change in a product, but it is not ready to be merged into flutter/plugins yet. For example: e2e tests for this feature are still missing.

I wanted to open this PR anyway to get early feedback. I have seen @amirh reply to earlier PR's regarding this feature. I hope you can give some feedback on this approach?

For example; should we make loadUrl and loadData throw an exception when the other one was used before? We can keep that knowledge on the dart side and just throw a dart exception, or should it be tried and possibly return an error depending on the platform implementation handles it?

Related Issues

This fixes flutter/flutter#32827

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[ahyenz]

Nice

[CanRau]

That'd be amazing 👏

[GeertJohan]

I've rebased this feature-branch on the latest master. Please review :)

[GeertJohan]

I think #3214 is better and should be merged instead of this.

[stuartmorgan-g]

Thanks for the submission! We apologize for the long delay in triaging this PR. We’re in the process of overhauling our PR triage system to respond much more quickly, as well as working through the backlog.

For example; should we make loadUrl and loadData throw an exception when the other one was used before?

Can you clarify the question here? What is the motivation for this exception?

Please review :)

As you noted originally, this PR isn't ready for review yet as it's missing tests. It looks like it should be relatively straightforward to review once they are ready though.

[stuartmorgan-g]

/cc @mvanbeusekom

[stuartmorgan-g]

Thank you for your contribution. I'm going to close this PR for now since there are outstanding comments, just to get this off our PR review queue. Please don't hesitate to submit a new PR if you have the time to address the review comments. Thanks!