depend on 2.10 branch for sdk packages (#150)

diff --git a/pubspec.yaml b/pubspec.yaml
index d9f2508..e45737f 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -22,12 +22,14 @@
     git:
       url: git://github.com/dart-lang/sdk.git
       path: pkg/js
+      ref: 2-10-pkgs
   matcher:
     git: git://github.com/dart-lang/matcher.git
   meta:
     git:
       url: git://github.com/dart-lang/sdk.git
       path: pkg/meta
+      ref: 2-10-pkgs
   path:
     git: git://github.com/dart-lang/path.git
   pedantic: