Add Context cancellation to clock.

common/clock/tags_test.go

Index: common/clock/tags_test.go
diff --git a/common/clock/tags_test.go b/common/clock/tags_test.go
new file mode 100644
index 0000000000000000000000000000000000000000..89ce227622a262c498a200805e70d99d98c97d6b
--- /dev/null
+++ b/common/clock/tags_test.go
@@ -0,0 +1,39 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package clock
+
+import (
+	"testing"
+
+	"golang.org/x/net/context"
+
+	. "github.com/smartystreets/goconvey/convey"
+)
+
+func TestTags(t *testing.T) {
+	Convey(`An empty Context`, t, func() {
+		c := context.Background()
+
+		Convey(`Should have nil tags.`, func() {
+			So(Tags(c), ShouldBeNil)
+		})
+
+		Convey(`With tag, "A"`, func() {
+			c = Tag(c, "A")
+
+			Convey(`Should have tags {"A"}.`, func() {
+				So(Tags(c), ShouldResemble, []string{"A"})
+			})
+
+			Convey(`And another tag, "B"`, func() {
+				c = Tag(c, "B")
+
+				Convey(`Should have tags {"A", "B"}.`, func() {
+					So(Tags(c), ShouldResemble, []string{"A", "B"})
+				})
+			})
+		})
+	})
+}