Add context-aware "time" library wrapper.

go/src/infra/libs/clock/testtimer.go

Index: go/src/infra/libs/clock/testtimer.go
diff --git a/go/src/infra/libs/clock/testtimer.go b/go/src/infra/libs/clock/testtimer.go
new file mode 100644
index 0000000000000000000000000000000000000000..d8e98b3df8e2438ff9a5f806f54f09397dcac535
--- /dev/null
+++ b/go/src/infra/libs/clock/testtimer.go
@@ -0,0 +1,52 @@
+// Copyright (c) 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 (
+	"time"
+)
+
+// TestTimer is a Timer implementation that can be used for testing. The timer can be
+// signalled via its Signal method, which writes an an underlying buffered channel.
+//
+// The channel is buffered so it can be used without requiring a separate signalling
+// goroutine.
+type TestTimer interface {
+	Timer
+	Signal(time.Time) // Signal the test timer.
+}
+
+type testTimer struct {
+	signalC chan time.Time // Underlying signal channel.
+	active  bool           // If true, the timer is active.
+}
+
+// NewTestTimer returns a new, instantiated TestTimer.
+func NewTestTimer() TestTimer {
+	return &testTimer{
+		signalC: make(chan time.Time, 1),
+	}
+}
+
+// Signals the timer.
+func (t *testTimer) Signal(tm time.Time) {
+	t.signalC <- tm
+}
+
+// Implements Timer.
+func (t *testTimer) GetC() (c <-chan time.Time) {
+	return t.signalC
+}
+
+// Implements Timer.
+func (t *testTimer) Reset(d time.Duration) bool {
+	return t.active
+}
+
+// Implements Timer.
+func (t *testTimer) Stop() (active bool) {
+	active, t.active = t.active, false
+	return
+}