Tweak SDK constraint checking a bit.

utils/pub/sdk_source.dart

Index: utils/pub/sdk_source.dart
diff --git a/utils/pub/sdk_source.dart b/utils/pub/sdk_source.dart
index 124d2651bb1cd76d066ac412e0ff67a05bcd133a..6fbba0959441d2740337dd91a63cff79fe869722 100644
--- a/utils/pub/sdk_source.dart
+++ b/utils/pub/sdk_source.dart
@@ -24,10 +24,10 @@ class SdkSource extends Source {
     return defer(() {
       var packageDir = _getPackagePath(id);
       // TODO(rnystrom): What if packageDir is null?
-      var package = new Package(id.name, packageDir, systemCache.sources);
+      var pubspec = new Pubspec.load(id.name, packageDir, systemCache.sources);
       // Ignore the pubspec's version, and use the SDK's.
-      return new Pubspec(id.name, sdk.version, package.pubspec.dependencies,
-          package.pubspec.environment);
+      return new Pubspec(id.name, sdk.version, pubspec.dependencies,
+          pubspec.environment);
     });
   }