Tweak SDK constraint checking a bit.

utils/pub/package.dart

Index: utils/pub/package.dart
diff --git a/utils/pub/package.dart b/utils/pub/package.dart
index ac9cc39703353a377d922bc8d4dbcf6a23772981..5485b65e78b33eb9228f5343a6183b93f4d861d9 100644
--- a/utils/pub/package.dart
+++ b/utils/pub/package.dart
@@ -58,26 +58,9 @@ class Package {
   /// Loads the package whose root directory is [packageDir]. [name] is the
   /// expected name of that package (e.g. the name given in the dependency), or
   /// `null` if the package being loaded is the entrypoint package.
-  factory Package(String name, String packageDir, SourceRegistry sources) {
-    var pubspecPath = 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 new Package._(packageDir, pubspec);
-    } on FormatException catch (ex) {
-      throw 'Could not parse $pubspecPath:\n${ex.message}';
-    }
-  }
+  Package.load(String name, String packageDir, SourceRegistry sources)
+      : dir = packageDir,
+        pubspec = new Pubspec.load(name, packageDir, sources);
 
   /// Constructs a package with the given pubspec. The package will have no
   /// directory associated with it.