Support Object Deconstruction or Else Statements

[mcmah309]

Given

sealed class ABC {}

final class A implements ABC {
  final int a;

  const A(this.a);
}

final class B implements ABC {
  final int b;

  const B(this.b);
}

final class C implements ABC {
  final int c;

  const C(this.c);
}

class Foo{
  final int bar;
  
  const Foo(this.bar);
}

Dart already supports object deconstruction:

void main(){
    final Foo(:bar) = Foo(1);
    //  can now use "bar"
}

And Dart supports If-Case statements:

void main(){
    ABC abc = A(1);
    if(abc case A(:final a)){
        // can now use "a"
    }
}

Therefore Dart should also support Object Deconstruction or Else Statements:

void main(){
    ABC abc = A(1);
    A(:final a) = abc else {
        // break, continue, return, or throw
    }
    // can now use "a"
}

This is effectively a pattern matching guard clause.

[mcmah309]

For reference, from another language: here

[water-mizuu]

This seems similar to #2703. You can see the discussion there.

[mcmah309]

Similar, but different. This is specifically for object deconstruction and fits well into the existing syntax and does not require reserving a new keyword like "let". If "if-case" statements are one side of the coin, then "Object Deconstruction or Else" is the other side of the coin.
Example with current syntax:

void main(){
  ABC abc = A(1);
  int a;
  if(abc case A(a:final innerA)){
    a = innerA;
  }
  else {
    return;
  }
  // use a
}

Example with proposed syntax:

void main(){
  ABC abc = A(1);
  A(:final a) = abc else {
    return;
  }
  // use a
}

[munificent]

I would like to have something like Swift's "guard let" or Rusts "let if" as well. I'm going to merge this issue with #2537, which discusses that more generally without being pinned to a specific syntax.