Allow to declare fields as 'external' for simpler JS interop.

[drecdroid]

Currently you have to write both getter and setter to have a field access an external JS field.
There are maybe some language restrinctions that don't allow fields as external but an annotation could solve this problem.

Now:

class CircularChartData{
  ...
  external String get color;
  external set color(String v);
  ...
}

Proposal:

class CircularCharData{
  ...
  external String color;
  ...
}

or:

class CircularCharData{
  ...
  @external String color;
  ...
}

or:

class CircularCharData{
  ...
  external String accessor color;
  ...
}

[lrhn]

I have no conceptual problem with allowing an "external field" as a shorthand for a pair of external getter and setter.
Since it is "external", it shouldn't introduce the normal field storage, only the signatures of the getters and setters. You can't initialize such a field in a generative constructor, so it's basically equivalent to implementing an interface with a field signature, and expecting the actual implementation of the getter/setter to come from somewhere externally.

[alexmarkov]

Related: #42560.