Add support for specifying SDK version constraints in pubspecs.

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 7129a7b45bc420f924b23fd04dd983162f3654a5..e0782b08cc3d0494c96f2d9c8b2d4b89c70ddeeb 100644
--- a/utils/pub/sdk_source.dart
+++ b/utils/pub/sdk_source.dart
@@ -25,7 +25,8 @@ class SdkSource extends Source {
       return Package.load(id.name, packageDir, systemCache.sources);
     }).then((package) {
       // Ignore the pubspec's version, and use the SDK's.
-      return new Pubspec(id.name, sdk.version, package.pubspec.dependencies);
+      return new Pubspec(id.name, sdk.version, package.pubspec.dependencies,
+          package.pubspec.environment);
     });
   }