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

utils/pub/entrypoint.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library entrypoint;
 
 import 'dart:async';
 import 'io.dart';
 import 'lock_file.dart';
 import 'log.dart' as log;
@@ skipping 139 matching lines @@
       group.add(new Future.immediate(root.pubspec));
 
       visitPackage(Pubspec pubspec) {
         for (var ref in pubspec.dependencies) {
           if (visited.contains(ref.name)) continue;
 
           // Look up the concrete version.
           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;
     });
   }
 
   /// Validates that the current Dart SDK version matches the SDK constraints
   /// of every package in the dependency graph. If a package's constraint does
   /// not match, prints an error.
   Future validateSdkConstraints() {
     return walkDependencies().then((pubspecs) {
@@ skipping 107 matching lines @@
 
   /// Creates a symlink to the `packages` directory in [dir] if none exists.
   Future _linkSecondaryPackageDir(String dir) {
     return defer(() {
       var to = join(dir, 'packages');
       if (entryExists(to)) return;
       return createSymlink(path, to);
     });
   }
 }