Add method returning all sub-routes of a Route object

[raptaml]

An easy way to get all sub-routes of a Route was missing. This PR adds it to abstract class Route and its implementation. Tests pass in current content-shell full-stable-45692.0.

[raptaml]

You are right. Hope that fits.

[pavelgj]

Also add the method to RouteHandle https://github.com/angular/route.dart/blob/master/lib/route_handle.dart

[raptaml]

Would you like to have RouteHandle return its route's subRoutes or throw "unsupported"? Think it should throw.

[pavelgj]

It should return _route.subRoutes

[raptaml]

Hi, will this be released? And if so, when is ETA ? Thanks