Add runtime type check for 'await' in DDC

[alexmarkov]

This is a DDC part of #49396.

According to the spec, await e should check runtime type of e to be a Future<flatten(S)> where S is a static type of e before waiting for e (otherwise it should wait for Future.value(e)). See #49396 for details.

Kernel AST contains the type to check in AwaitExpression.runtimeCheckType.

@sigmundch @nshahan

[nshahan]

Waiting for test cases to be finalized and landed https://dart-review.git.corp.google.com/c/sdk/+/267422.

[nshahan]

Tests landed but contain unexpected compile time errors. It sounds like these should not be errors. Waiting on CFE work to accept the code in the test.

[alexmarkov]

You can also use language/async/await_type_check_test which was landed in 66f1dee until those compile-time errors are sorted out.

[eernstg]

@alexmarkov and @nshahan, the definition of flatten turned out to be too aggressive (causing gratuitous loss of type information), so it was adjusted, cf. #49396 (comment). Sorry about the inconvenience this may cause!

[nshahan]

@alexmarkov @eernstg Thanks for the updates!