Validate packages against their SDK constraints.

utils/pub/pubspec.dart

Index: utils/pub/pubspec.dart
diff --git a/utils/pub/pubspec.dart b/utils/pub/pubspec.dart
index 11b840e859b8e917a756a1a67430f67c35c9b217..492acd36c991eb2bf674670d54f7047ebdc7290c 100644
--- a/utils/pub/pubspec.dart
+++ b/utils/pub/pubspec.dart
@@ -4,12 +4,13 @@
 
 library pubspec;
 
+import '../../pkg/yaml/lib/yaml.dart';
+
 import 'package.dart';
 import 'source.dart';
 import 'source_registry.dart';
 import 'utils.dart';
 import 'version.dart';
-import '../../pkg/yaml/lib/yaml.dart';
 
 /// The parsed and validated contents of a pubspec file.
 class Pubspec {
@@ -44,6 +45,7 @@ class Pubspec {
   bool get isEmpty =>
     name == null && version == Version.none && dependencies.isEmpty;
 
+  // TODO(rnystrom): Make this a static method to match corelib.
   /// Parses the pubspec whose text is [contents]. If the pubspec doesn't define
   /// version for itself, it defaults to [Version.none].
   factory Pubspec.parse(String contents, SourceRegistry sources) {