Tweak SDK constraint checking a bit.

utils/pub/pubspec.dart

Index: utils/pub/pubspec.dart
diff --git a/utils/pub/pubspec.dart b/utils/pub/pubspec.dart
index 492acd36c991eb2bf674670d54f7047ebdc7290c..f7c3579325c313111ec14cc705f20c18d7c7dd63 100644
--- a/utils/pub/pubspec.dart
+++ b/utils/pub/pubspec.dart
@@ -5,7 +5,9 @@
 library pubspec;
 
 import '../../pkg/yaml/lib/yaml.dart';
+import '../../pkg/path/lib/path.dart' as path;
 
+import 'io.dart';
 import 'package.dart';
 import 'source.dart';
 import 'source_registry.dart';
@@ -30,6 +32,28 @@ class Pubspec {
   /// are derived.
   final Map<String, Object> fields;
 
+  /// Loads the pubspec for a package [name] located in [packageDir].
+  factory Pubspec.load(String name, String packageDir, SourceRegistry sources) {
+    var pubspecPath = path.join(packageDir, 'pubspec.yaml');
+    if (!fileExists(pubspecPath)) throw new PubspecNotFoundException(name);
+
+    try {
+      var pubspec = new Pubspec.parse(readTextFile(pubspecPath), sources);
+
+      if (pubspec.name == null) {
+        throw new PubspecHasNoNameException(name);
+      }
+
+      if (name != null && pubspec.name != name) {
+        throw new PubspecNameMismatchException(name, pubspec.name);
+      }
+
+      return pubspec;
+    } on FormatException catch (ex) {
+      throw 'Could not parse $pubspecPath:\n${ex.message}';
+    }
+  }
+
   Pubspec(this.name, this.version, this.dependencies, this.environment,
       [Map<String, Object> fields])
     : this.fields = fields == null ? {} : fields;