Obtaining source span for DartObject (computeConstantValue)

[matanlurey]

(Reference issue is angulardart/angular#1350)

tl;dr: AngularDart has a metadata-based API for consumers:

import 'package:angular/angular.dart';

@Component(
  selector: 'bar',
)
class Foo {}

Imagine we wanted to validate that selector was always the class name, kabob-cased. The best we can do today is present either a source span for class Foo or a source span for the entire @Component annotation - of which it can become quite large for complex components.

We'd like to be able to get a source span so we could present:

@Component(
  selector: 'bar',
  ^^^^^^^^^^^^^^^
  Expected 'foo' for 'class Foo', but got 'bar'

Unfortunately it looks like we lack the APIs in package:analyzer to do this. We could re-parse the file, and try to "recompute" what the source span would be, but that would likely be non-trivial and require a lot of ugly code. Any suggestions please?