Fix some static warnings.

sdk/lib/_internal/pub/lib/src/source/git.dart

Index: sdk/lib/_internal/pub/lib/src/source/git.dart
diff --git a/sdk/lib/_internal/pub/lib/src/source/git.dart b/sdk/lib/_internal/pub/lib/src/source/git.dart
index 42e6856837da1d8ab5bb087fd339185a66a8363a..5a7f5e5977b2b0cfeab7be96d29a9a4c0b489417 100644
--- a/sdk/lib/_internal/pub/lib/src/source/git.dart
+++ b/sdk/lib/_internal/pub/lib/src/source/git.dart
@@ -48,11 +48,11 @@ class GitSource extends Source {
       return _ensureRepoCache(id);
     }).then((_) => systemCacheDirectory(id)).then((path) {
       revisionCachePath = path;
-      if (entryExists(revisionCachePath)) return;
+      if (entryExists(revisionCachePath)) return null;
       return _clone(_repoCachePath(id), revisionCachePath, mirror: false);
     }).then((_) {
       var ref = _getEffectiveRef(id);
-      if (ref == 'HEAD') return;
+      if (ref == 'HEAD') return null;
       return _checkOut(revisionCachePath, ref);
     }).then((_) {
       return new Package.load(id.name, revisionCachePath, systemCache.sources);