Decouple PLS from MGS.

service/datastore/datastore_test.go

Index: service/datastore/datastore_test.go
diff --git a/service/datastore/datastore_test.go b/service/datastore/datastore_test.go
index d86123f523ef16a5d349cf9ebccd4c9580f9f1a4..f60da23cd2530f144bd291410d3022cc052a419d 100644
--- a/service/datastore/datastore_test.go
+++ b/service/datastore/datastore_test.go
@@ -241,6 +241,28 @@ func (f *FakePLS) Problem() error {
 	return nil
 }
 
+type MGSWithNoKind struct {
+	S string
+}
+
+func (s *MGSWithNoKind) GetMetaDefault(key string, dflt interface{}) interface{} {
+	return GetMetaDefaultImpl(s.GetMeta, key, dflt)
+}
+
+func (s *MGSWithNoKind) GetMeta(key string) (interface{}, error) {
+	return nil, ErrMetaFieldUnset
+}
+
+func (s *MGSWithNoKind) GetAllMeta() PropertyMap {
+	return PropertyMap{}
+}
+
+func (s *MGSWithNoKind) SetMeta(key string, val interface{}) error {
+	return ErrMetaFieldUnset
+}
+
+var _ MetaGetterSetter = (*MGSWithNoKind)(nil)
+
 func TestKeyForObj(t *testing.T) {
 	t.Parallel()
 
@@ -376,6 +398,11 @@ func TestPut(t *testing.T) {
 				cs := CommonStruct{}
 				So(ds.Put(cs).Error(), ShouldContainSubstring, "invalid Put input type")
 			})
+
+			Convey("struct with no $kind is an error", func() {
+				s := MGSWithNoKind{}
+				So(ds.Put(&s).Error(), ShouldContainSubstring, "unable to extract $kind")
+			})
 		})
 
 		Convey("ok", func() {