[firebase_core_web] Add initial implementation

[harryterkelsen]

Description

This adds web support to firebase_core_web.

Related Issues

• Fixes [firebase_core] Support firebase_core on the web. flutter/flutter#43940

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[ditman]

Last time I checked, we didn't want people using dart:js/dart:js_util directly unless they were in the places where you can't use package: deps. I think you should port this to using package:js deps (I suggested a couple of changes, that may help)

[harryterkelsen]

You're right. We should prefer package:js over dart:js for packages. Thanks for the review! PTAL

[ditman]

Do we need the dummy ios/firebase_core_web.podspec file in this package so we don't break compilation on IOS?

[ditman]

(There's also scripts similar to flutter/plugins so we might need to exclude the platform_interface from here: https://github.com/FirebaseExtended/flutterfire/blob/master/script/build_all_plugins_app.sh#L15 ?)

[harryterkelsen]

Added a dummy podspec. It looks like firebase_core is already excluded from the build_all_plugins script

[harryterkelsen]

Merging despite failing tests on stable since web plugins aren't supported on stable.