Add User service.

impl/dummy/dummy_test.go

Index: impl/dummy/dummy_test.go
diff --git a/impl/dummy/dummy_test.go b/impl/dummy/dummy_test.go
index 72630ae189f7aba0b7c2e70fed59b47edef9a599..40057489571181932731734d0d2a43405470f8be 100644
--- a/impl/dummy/dummy_test.go
+++ b/impl/dummy/dummy_test.go
@@ -11,6 +11,7 @@ import (
 	infoS "github.com/luci/gae/service/info"
 	mcS "github.com/luci/gae/service/memcache"
 	tqS "github.com/luci/gae/service/taskqueue"
+	userS "github.com/luci/gae/service/user"
 	. "github.com/smartystreets/goconvey/convey"
 	"golang.org/x/net/context"
 )
@@ -72,5 +73,14 @@ func TestContextAccess(t *testing.T) {
 			}, ShouldPanicWith, "dummy: method TaskQueue.Purge is not implemented")
 		})
 
+		Convey("User", func() {
+			c = userS.Set(c, User())
+			So(userS.Get(c), ShouldNotBeNil)
+			So(func() {
+				defer p()
+				_ = userS.Get(c).IsAdmin()
+			}, ShouldPanicWith, "dummy: method User.IsAdmin is not implemented")
+		})
+
 	})
 }