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

utils/pub/system_cache.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 system_cache;
 
 import 'dart:io';
 import 'dart:async';
 
 import 'git_source.dart';
@@ skipping 45 matching lines @@
   /// Registers a new source. This source must not have the same name as a
   /// source that's already been registered.
   void register(Source source) {
     source.bind(this);
     sources.register(source);
   }
 
   /// Gets the package identified by [id]. If the package is already cached,
   /// reads it from the cache. Otherwise, requests it from the source.
   Future<Pubspec> describe(PackageId id) {
+    if (id.isRoot) throw new ArgumentError("Cannot describe the root package.");
+
     // Try to get it from the system cache first.
     if (id.source.shouldCache) {
       return id.systemCacheDirectory.then((packageDir) {
         if (!dirExists(packageDir)) return id.describe();
         return new Pubspec.load(id.name, packageDir, sources);
       });
     }
 
     // Not cached, so get it from the source.
     return id.describe();
@@ skipping 32 matching lines @@
 
   /// Delete's the system cache's internal temp directory.
   Future deleteTempDir() {
     log.fine('Clean up system cache temp directory $tempDir.');
     return defer(() {
       if (!dirExists(tempDir)) return;
       return deleteDir(tempDir);
     });
   }
 }