Missing type definitions in `webcryptoapi.dart`

[Schwusch]

In 1.0.0, some SubtleCrypto methods have untyped parameters, thus making them hard to use:
E.g. importKey():

Before 1.0.0, it was possible to figure out and pass KeyAlgorithm(name: 'AES-GCM') as the algorithm argument. This class is however gone in 1.0.0.
Other missing definition examples are AesGcmParams, which was useful for SubtleCrypto.encrypt()/ SubtleCrypto.decrypt().
Current implementation of AlgorithmIdentifier is a simple typedef:

typedef AlgorithmIdentifier = JSAny;

[srujzs]

Hmm, this may be a bug with the Web IDL but not sure.

KeyAlgorithm is defined but never used in the IDL except as a superinterface: https://github.com/w3c/webref/blob/5308f9a9c0914d765fcbbb55f7d01d313ae37eda/ed/idl/WebCryptoAPI.idl#L25. The subinterfaces are never referred to either.

7604578 uses the MDN compatibility data to avoid emitting experimental/nonstandard APIs. MDN doesn't contain compatibility data for dictionaries, so we depend on whether or not its used by a non-experimental/standard API to determine if we should emit the dictionary. Here, KeyAlgorithm is never actually used except as a superinterface, and the subinterfaces are never used either: https://github.com/w3c/webref/blob/5308f9a9c0914d765fcbbb55f7d01d313ae37eda/ed/idl/WebCryptoAPI.idl#L25. The case is the same with AesGcmParams: https://github.com/w3c/webref/blob/5308f9a9c0914d765fcbbb55f7d01d313ae37eda/ed/idl/WebCryptoAPI.idl#L206.

We could instead choose to only not emit dictionaries that are explicitly used by experimental/nonstandard APIs, and that may cover this, but it also may mean we're including some dictionary types that are subject to change often.