Handle circular dependencies on the root package when checking SDK constraints.

utils/pub/entrypoint.dart

Index: utils/pub/entrypoint.dart
diff --git a/utils/pub/entrypoint.dart b/utils/pub/entrypoint.dart
index 843d41d2a753d59dc632049c85517488580566e6..6d7ffe09f860f63b5713e3958d8f8ea601fb71bb 100644
--- a/utils/pub/entrypoint.dart
+++ b/utils/pub/entrypoint.dart
@@ -157,13 +157,19 @@ class Entrypoint {
           var id = lockFile.packages[ref.name];
 
           visited.add(ref.name);
-          var future = cache.describe(id);
+          var future;
+          if (ref.name == root.name) {
+            future = new Future<Pubspec>.immediate(root.pubspec);
+          } else {
+            future = cache.describe(id);
+          }
           group.add(future.then(visitPackage));
         }
 
         return pubspec;
       }
 
+      visited.add(root.name);
       visitPackage(root.pubspec);
       return group.future;
     });