[dart2wasm] Move web wasm embedder to new dart2wasm embedder API

[mkustermann]

This removes usages of the deprecated top-level instantiate and invoke methods and replaces it by method calls to the embedder objects.

See [0] for the change that introduce this embedder API.

[0] https://dart-review.googlesource.com/c/sdk/+/383242/6

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[eyebrowsoffire]

LGTM

[mkustermann]

Thanks, @eyebrowsoffire & @osa1 !