Implement Mail 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 40057489571181932731734d0d2a43405470f8be..34ff57986e84ebec819467c9886373d36985e60d 100644
--- a/impl/dummy/dummy_test.go
+++ b/impl/dummy/dummy_test.go
@@ -9,6 +9,7 @@ import (
 
 	dsS "github.com/luci/gae/service/datastore"
 	infoS "github.com/luci/gae/service/info"
+	mailS "github.com/luci/gae/service/mail"
 	mcS "github.com/luci/gae/service/memcache"
 	tqS "github.com/luci/gae/service/taskqueue"
 	userS "github.com/luci/gae/service/user"
@@ -82,5 +83,14 @@ func TestContextAccess(t *testing.T) {
 			}, ShouldPanicWith, "dummy: method User.IsAdmin is not implemented")
 		})
 
+		Convey("Mail", func() {
+			c = mailS.Set(c, Mail())
+			So(mailS.Get(c), ShouldNotBeNil)
+			So(func() {
+				defer p()
+				_ = mailS.Get(c).Send(nil)
+			}, ShouldPanicWith, "dummy: method Mail.Send is not implemented")
+		})
+
 	})
 }