[webview_flutter] Proof of concept web implementation

[stuartmorgan-g]

Initial proof of concept implementation of flutter_webview for the web.

This is #4565 with the following changes:

• Fixes unit test compilation on master
• Adds very minimal integration tests

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[ditman]

This is basically the same as #4565, but with a small fix to Mocks and a couple of new tests added by @stuartmorgan.

[a-eniou-pvotal]

@stuartmorgan As I can see, WebViewPlatformController is not supported, I wonder if it will be in the next release?
https://github.com/flutter/plugins/blob/master/packages/webview_flutter/webview_flutter_web/lib/webview_flutter_web.dart

[stuartmorgan-g]

@stuartmorgan As I can see, WebViewPlatformController is not supported

I don't know what that means, given that you provided a link to a file that implements it. But if you have bugs or feature requests for the package, please use the issue tracker as always.

[a-eniou-pvotal]

@stuartmorgan I mean most of functions throw UnimplementedError();
Ok I will use issue trackers in the future