Rule Req: recommend using a base type instead of var when LUB applies

[robbecker-wf]

I would like to get a lint warning/hint for ternary operators when encountering code like this:

A -> SubClass1 -> BaseClass, with Element -> Object
B -> SubClass2 -> BaseClass, with Element -> Object

var shouldBeOfTypeBaseClass = someBoolCheck() ? new A() : new B();
// the analyzer will infer that shouldBeOfTypeBaseClass is of type Object instead of the 
// desired `BaseClass` class

// the same thing would apply to the ?? operator
A a;
B b = new B();
var shouldBeOfTypeBaseClass = a ?? b;

See #30079 (comment) for more info on motivation.

I'd like to see a lint warning on the var before shouldBeOfTypeBaseClass pointing out that the type of shouldBeOfTypeBaseClass will be inferred as Object and you should specify a base class type instead of using var.

[bwilkerson]

@leafpetersen Another data point to consider when looking at LUB.

[alexeieleusis]

What does LUB stands for?

[zoechi]

Least Upper Bound dart-archive/dev_compiler#288

[leafpetersen]

Noted.