Make the tearOff return the correct function instead of a closure that returns the created object

[jodinathan]

I am playing with webcomponents in Dart.
One way to be able to use webcomponents is making the underlying js class extend the implemented HTMLElement: #48261

Another way was if the tearOff in JS was the function with the constructor calls inside of it.

Example:

class Foo implements HTMLElement {
  Foo() {
    print('Foo constructor!');
  }
}

void main () {
  print(Foo.new);
}

Today this results in this JS:

   Foo___new_tearOff() { // the tearOff here is in reality a function that creates the object
      A.print("Foo constructor!");
      return new A.Foo(); 
    },
    Foo: function Foo() { // we don't really have access to this
       // also, the constructor calls are outside this function
    }

There is a high change that WebComponents could work if the js result was something like this:

    Foo: function Foo() {
      A.print("Foo constructor!"); // remove the constructor calls from the Foo___new_tearOff and make it inside 
     // the correct constructor function
    },
   Foo___new_tearOff: A.Foo, // let the tearOff really return the function callback

Then we can use Reflect.construct(HTMLElement, [], Foo___new_tearOff) in webcomponents