Proposal: factor out `Geometry` class declarations

[nate-thegrate]

Assumption

This issue assumes that Dart will have a convenient dot syntax in the somewhat near future:

Padding(
  padding: .only(left: 42.0),
)

Problem

Both EdgeInsets and EdgeInsetsDirectional have a .only() constructor; there would need to be a way to differentiate between the two.

Proposal

Change the class structure.

Before

abstract class EdgeInsetsGeometry {}

class EdgeInsets extends EdgeInsetsGeometry {}

class EdgeInsetsDirectional extends EdgeInsetsGeometry {}

After

@Deprecated()
typedef EdgeInsetsGeometry = EdgeInsets;

class EdgeInsets {}

class EdgeInsetsDirectional extends EdgeInsets {}

And then .only() would hopefully be interpreted as EdgeInsets.only() by default. (The same would be done with AlignmentGeometry and BorderRadiusGeometry as well.)

I believe this should be a mostly trivial/non-breaking transition, with one catch:

void foo(EdgeInsetsDirectional directionalPadding) {
  print(directionalPadding is EdgeInsets); // would change from "false" to "true"
}

[nate-thegrate]

(forgot I need to unassign for this to show up in triage 😄)