Differentiate between single- and multi- props.

service/datastore/multiarg.go

 // Copyright 2015 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package datastore
 
 import (
         "fmt"
         "reflect"
 
@@ skipping 271 matching lines @@
 //
 // GetMeta will be implemented, returning the *Key for the "key" meta.
 //
 // If slot is addressable, SetMeta will allow it to be set to the supplied
 // Value.
 type keyMGS struct {
         slot reflect.Value
 }
 
 func (mgs *keyMGS) GetAllMeta() PropertyMap {
-        return PropertyMap{"$key": []Property{MkPropertyNI(mgs.slot.Interface())}}
+        return PropertyMap{"$key": MkPropertyNI(mgs.slot.Interface())}
 }
 
 func (mgs *keyMGS) GetMeta(key string) (interface{}, bool) {
         if key != "key" {
                 return nil, false
         }
         return mgs.slot.Interface(), true
 }
 
 func (mgs *keyMGS) SetMeta(key string, value interface{}) bool {
@@ skipping 262 matching lines @@
         default:
                 // Pass through to track as MultiError.
                 bt.errorTracker.trackError(it, err)
         }
 }
 
 func (bt *boolTracker) result() *ExistsResult {
         bt.res.updateSlices()
         return &bt.res
 }