Least upper bound can produce malbounded types.

Noting in passing for future consideration. Not a soundness issue, since we already allow generic arguments to violate their bounds in types, but an unpleasantness. Example below demonstrates the issue: upper bound computation produces `A` for the type of `x`, which is not a well-bounded type.

```dart
class A0 {}

class B0 {}
class B1 extends B0 {}

class C extends A0 implements B1 {}
class D extends A0 implements B1 {}

class A {}

foo(int z) {
A ac;
A ad;
var x = z == 3 ? ac : ad;
z = x;
}
```

Issue #651 provides a Dart reproducer for least-upper-bound computation; start by locating the language specification and type-inference tests covering conditional expressions and generic bounds. The report names no files or tests, and done requires an agreed behavior plus specification or regression-test coverage for the malbounded inferred type.

Written by the indexing model from the issue text.