Fix callback refs with non-dynamic args failing in Dart 2

[greglittlefield-wf]

Motivation

Callback refs with non-dynamic arguments weren't being handled properly, and would result in obtuse error messages.

Example:

• dynamic ref argument (works):

  Foo({'ref': (ref) => _fooRef = ref})

• non-dynamic ref argument (does not work):

  Foo({'ref': (FooComponent ref) => _fooRef = ref})

In DDC, the ref function would be called correctly, but the ref argument would be of type NativeJavaScriptObject instead of the expected component class. This is because the JS component was being passed instead of the Dart one.

In dart2js, React would throw upon creation of the ReactElement, with the following error.

Invariant Violation: addComponentAsRefTo(...): Only a ReactOwner can have refs. You might be adding a ref to a component that was not created inside a component's render method, or you have multiple copies of React loaded (details: https://fb.me/react-refs-must-have-owner).

This is because allowInterop was not called on the provided function.

Solution

• Fix type checking using Null arity hack shown here: https://github.com/dart-lang/language/issues/3758
• Add tests

Testing

• Verify tests pass in both DDC and dart2js
• Verify tests fail in both DDC and dart2js if either of the following changes are made in react_client.dart:
  • changing the is _CallbackRef<Null> check to is _CallbackRef<dynamic>
  • removing the as dynamic cast in the ref invocation

[aaronstgeorge-wf]

Nice nice nice nice

[smaifullerton-wk]

QA +1

• [x] Verify tests pass in both DDC and dart2js
• [x] Verify tests fail in both DDC and dart2js if either of the following changes are made in react_client.dart:
• [x] changing the is _CallbackRef check to is _CallbackRef
• [x] removing the as dynamic cast in the ref invocation

[smaifullerton-wk]

+10 - typo fixes do not impact QA