Inconsistent inference in analyzer and CFE

The following code is inferred inconsistently between the CFE and the analyzer:

```dart
void main(){
String? variable;
var f = Future.value(variable);
Future x = f;
}
```

The analyzer emits no error, suggesting that inference is inferring `String` for the type argument to `Future.value`, whereas the CFE emits the following error:

```
leafp-macbookpro2:sdk leafp$ ~/src/dart-repo/sdk/xcodebuild/ReleaseARM64/dart ~/tmp/example.dart
../../../tmp/example.dart:5:22: Error: A value of type 'Future' can't be assigned to a variable of type 'Future' because 'String?' is nullable and 'String' isn't.
- 'Future' is from 'dart:async'.
Future x = f;
```

I believe the analyzer behavior here is in general more useful, I'm not sure off hand which is consistent with the spec. cc @stereotype441 @eernstg @johnniwinther @scheglov