Add support for dart2wasm+Safari

[eyebrowsoffire]

This PR attempts to enable dart2wasm testing with Safari. For whatever reason, in Safari, the message.source.location?.href check on the message that sends the port from the iframe to the host window actually returns the parent frame rather than the iframe's href, and only when dart2wasm is used. This changes the logic to explicitly add this value to the message being sent to avoid this problem.

This fixes #2482

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Changelog Entry ❗

Package	Changed Files
package:test	pkgs/test/lib/dart.js pkgs/test/lib/src/runner/browser/post_message_channel.dart pkgs/test/lib/src/runner/browser/static/host.dart.js
package:test_api	pkgs/test_api/lib/src/backend/runtime.dart

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[natebosch]

This change is not backwards compatible. A compatible host.dart needs to be rolled out everywhere before we can land this change.

[eyebrowsoffire]

I suppose maybe I could post two messages, one that is just the 'port' string and the other that's the new structured message?

[eyebrowsoffire]

Hmm, that won't work because we're transferring the port.