Null checks are incorrectly optimized out for Flutter Border lerp function

[ferhatb]

Original customer issue: flutter/flutter#63740

Code that reproduces problem:
https://gist.github.com/rydmike/2e8e6a4cc9126dbe6f95e81e674f6d6b

The following BoxBorder.lerp/Border.lerp function is compiled from:

static Border? lerp(Border? a, Border? b, double t) {
    assert(t != null);
    if (a == null && b == null)
      return null;
    if (a == null)
      return b!.scale(t);
    if (b == null)
      return a.scale(1.0 - t);
    return Border(
      top: BorderSide.lerp(a.top, b.top, t),
      right: BorderSide.lerp(a.right, b.right, t),
      bottom: BorderSide.lerp(a.bottom, b.bottom, t),
      left: BorderSide.lerp(a.left, b.left, t),
    );
  }

// The caller
 static BoxBorder? lerp(BoxBorder? a, BoxBorder? b, double t) {
    assert(t != null);
    if ((a is Border?) && (b is Border?))
      return Border.lerp(a, b, t);
.......

to:

Border_lerp: function(a, b, t) {
  ///!!!!! NO null checks
      return new F.Border(Y.BorderSide_lerp(a.top, b.top, t), Y.BorderSide_lerp(a.right, b.right, t), Y.BorderSide_lerp(a.bottom, b.bottom, t), Y.BorderSide_lerp(a.left, b.left, t));
    },


BoxBorder_lerp: function(a, b, t) {
      var t0, t2,
        t1 = type$.nullable_Border;
      if (t1._is(a) && t1._is(b))
        return F.Border_lerp(a, b, t);
      t1 = type$.nullable_BorderDirectional;

[rakudrama]

It seems that global type analysis is incorrectly strengthening nullable type tests.
Smaller example:

String upcase(String? s) {
  if (s == null) return '';
  return s.toUpperCase();
}

String format(dynamic thing) {
  if (thing is String?) return upcase(thing);
  if (thing is num) return '$thing';
  return '?';
}

main() {
  print(format(null));
  print(format('hello'));
  print(format([]));
}

  Z = {
    upcase: function(s) {
      return s.toUpperCase();
    },
    format: function(thing) {
      if (type$.nullable_String._is(thing))
        return Z.upcase(thing);
      return "?";
    },

[rydmike]

Like I mentioned in flutter/flutter#65358, I think it is possible that this issue is having an impact in more areas than what has yet been reported or fully detected with Flutter master/dev channel Web release builds. At least this particular case seems to be related: flutter/flutter#64960.

I have also seen similar issues with other Flutter's implicit animated widgets (e.g. AnimatedSwitcher) and also with Animated theme, that all use a lot of lerps and the Themes + Sub themes that all depend on null for default values, so it is a possibility that this might be the cause of those issues as well. Pleased to see it is getting looked into.

In current state I think it will break a lot more for Flutter Web Beta channel builds if this issue regresses into next beta release. I suspect I have just scratch the surface with these findings.

[pcsosinski]

@franklinyow FYI this is likely a blocker for 2.10

[franklinyow]

https://dart-review.googlesource.com/c/sdk/+/162247