Type narrowing with foo.constructor

TypeScript Version: 2.3.1

Code

class FooTask {
  foo: number;
}

class BarTask {
  bar: number;
}

type taskType = FooTask|BarTask;

function createEvent(task: taskType) {
  if (task instanceof FooTask) {
    task.foo; // Works.
  }

  switch (task.constructor) {
    case FooTask:
      task.foo; // Fails.
  }
}

It would be nice if the second worked like the first; narrowed the type of task appropriately.

Use the provided FooTask/BarTask example with TypeScript 2.3.1 to reproduce the difference between instanceof and task.constructor narrowing. Trace the compiler's type-narrowing path for the switch and compare it with the instanceof case. Done means task.foo is accepted in the FooTask branch without weakening the existing narrowing behavior.