Type promotion fails with null checks in Iterable.where()

If you have an `Iterable` and wish to filter it to an `Iterable` which only contains the non-null elements, using `Iterable.where()` does not end up promoting the type of `E?`.

For example, when processing an XML document, the following code

```
Iterable files = xmlElements
.map((XmlElement element) => element.getAttribute('href'))
.where((String? href) => href != null)
.map((String href) => File(path.dirname(xmlFile.path) + href));
```
results in the following error from the Dart Analyzer:`error: The argument type 'File Function(String)' can't be assigned to the parameter type 'dynamic Function(String?)'.`

It is worth noting that this approach does correctly promote the type.
```
Iterable files = xmlElements
.map((XmlElement element) => element.getAttribute('href'))
.whereType()
.map((String href) => File(path.dirname(xmlFile.path) + href));
```

However, I don't think all use cases are necessarily as easily worked around. The addition of an `Iterable.whereNotType()` would make cases of mixed-content Iterables easier to filter (`Iterable.whereType()` should be equivalent to that, but is clunkier to read the intent of I think). A need to filter to elements of either `A` OR `B` however, would necessitate the use of `Iterable.where()` without a ready type-promoting alternative that I can see.

Start by reproducing the Dart Analyzer error with the Iterable, where((String? href) => href != null), and map((String href) => ...) example. Compare it with the working whereType() example and review the discussion around type promotion and a possible whereNotType(). Done means the intended behavior or language change is clearly specified and the analyzer accepts the supported pattern.

Written by the indexing model from the issue text.