`dart2wasm`: Export constant identifying the compiler about sdk HOT 5 CLOSED

Need this here as well! Could be very useful for example projects for packages to show an indicator as to whether it's running with WASM, to verify potential performance improvements.

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As you link to #54785, this is something that can be useful for more than just Dart2Wasm.
I'd prefer a general solution to something that only works for one compiler.
(Preferably entries that distinguish, at least, whether running natively, in JS or in Wasm, and possibly also whether compiled in development mode or production mode, and whether asserts are enabled.)

That is, I'd want to also define dart.tool.vm, dart.tool.dart2js, dart.tool.dev_compiler (or whatever names they want to be known under).
I believe dart.tool.dart2js already exists. The rest should too.

(But I'm OK with just agreeing that they should all exist, and then landing this as the first step.)

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I'd prefer a general solution to something that only works for one compiler.

I'm not claiming this fixes #54785 :) I just need the identification in dart2wasm and I think it would be nice to have it in a format that will eventually be consistent between all tools.

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@lrhn is there a problem with adding dart.tool.dart2wasm now, without waiting for #54785?

It looks like dart2js added it ~9 months ago:

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I have no problem with landing this as-is. (That's what I tried to say in the last line of my previous comment, maybe I should just be more direct. Ship it!)

I don't know whether I'm the one who should get to decide this. So far, nobody else does, so I'll take it!

I hope that this format is what we end up on, and if we don't, we'll have to take it from there. Since Dart2JS already used it, it'll only a bigger migration problem, not a more difficult one.

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