Add simplest service/urlfetch implementation.

service/urlfetch/context.go

+// 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 urlfetch provides a way for an application to get http.RoundTripper
+// that can make outbound HTTP requests. If used for https:// protocol, will
+// always validate SSL certificates.
+package urlfetch
+
+import (
+        "net/http"
+
+        "golang.org/x/net/context"
+)
+
+type key int
+
+var serviceKey key
+
+// Factory is the function signature for factory methods compatible with
+// SetFactory.
+type Factory func(context.Context) http.RoundTripper
+
+// Get pulls http.RoundTripper implementation from context or
+// http.DefaultTransport if not set.
+func Get(c context.Context) http.RoundTripper {
+        if f, ok := c.Value(serviceKey).(Factory); ok && f != nil {
+                return f(c)
+        }
+        return http.DefaultTransport

[Vadim Sh., 2015/09/18 01:15:02]

no memory implementation for now because this covers it :-/

[iannucci, 2015/09/18 01:19:38]

but this means that it'll be making actual http access in tests? Or I
misunderstand?

[Vadim Sh., 2015/09/18 01:22:35]

Well... One way of testing http stuff is actualy via loopback interface: 
http://golang.org/pkg/net/http/httptest/ 

What I meant by "coverts it": it won't panic with used. Presumably unit tests
will use urlfetch.Set(mockedTransport).

Should it panic by default?

+}
+
+// SetFactory sets the function to produce http.RoundTripper instances,
+// as returned by the Get method.
+func SetFactory(c context.Context, f Factory) context.Context {
+        return context.WithValue(c, serviceKey, f)
+}
+
+// Set sets the current http.RoundTripper object in the context. Useful for
+// testing with a quick mock. This is just a shorthand SetFactory invocation
+// to set a factory which always returns the same object.
+func Set(c context.Context, r http.RoundTripper) context.Context {
+        return SetFactory(c, func(context.Context) http.RoundTripper { return r })
+}