Let's fix `if let` syntax

[David_Catmull]

@stevex

function plop(foo: Foo) {
  const x = foo.bar.length; // error, Foo.bar? is possibly undefined
  if (foo.bar != null) {
    const y = foo.bar.length; // this is fine
  }
}

Personally I'm not fond of this approach. It is convenient, but I find it really weird that merely performing a comparison would change the type of a variable from optional to non-optional.