Improve circular dependency overrides.

lib/src/solver/backtracking_solver.dart

Index: lib/src/solver/backtracking_solver.dart
diff --git a/lib/src/solver/backtracking_solver.dart b/lib/src/solver/backtracking_solver.dart
index 4735aada47c37880035c9dbabe6921b61b2132e9..dd46b751330090759a35468556e17dc92668834b 100644
--- a/lib/src/solver/backtracking_solver.dart
+++ b/lib/src/solver/backtracking_solver.dart
@@ -579,6 +579,13 @@ class BacktrackingSolver {
     } else {
       // Ignore any overridden dependencies.
       deps.removeWhere((dep) => _overrides.containsKey(dep.name));
+
+      // If an overridden dependencies depends on the root package, ignore that

[Bob Nystrom, 2016/08/25 00:30:02]

"dependencies" -> "dependency".

[nweiz, 2016/08/25 00:31:02]

Done.

+      // dependency. This ensures that users can work on the next version of one
+      // side of a circular dependency easily.
+      if (_overrides.containsKey(id.name)) {
+        deps.removeWhere((dep) => dep.name == root.name);
+      }
     }
 
     // Make sure the package doesn't have any bad dependencies.