diff --git a/client_appengine.go b/client_appengine.go
deleted file mode 100644
index 8962c49..0000000
--- a/client_appengine.go
+++ /dev/null
@@ -1,25 +0,0 @@
-// Copyright 2014 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// +build appengine
-
-// App Engine hooks.
-
-package oauth2
-
-import (
-	"net/http"
-
-	"golang.org/x/net/context"
-	"golang.org/x/oauth2/internal"
-	"google.golang.org/appengine/urlfetch"
-)
-
-func init() {
-	internal.RegisterContextClientFunc(contextClientAppEngine)
-}
-
-func contextClientAppEngine(ctx context.Context) (*http.Client, error) {
-	return urlfetch.Client(ctx), nil
-}
diff --git a/internal/client_appengine.go b/internal/client_appengine.go
new file mode 100644
index 0000000..7434871
--- /dev/null
+++ b/internal/client_appengine.go
@@ -0,0 +1,13 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build appengine
+
+package internal
+
+import "google.golang.org/appengine/urlfetch"
+
+func init() {
+	appengineClientHook = urlfetch.Client
+}
diff --git a/internal/token.go b/internal/token.go
index 558ce95..881fbef 100644
--- a/internal/token.go
+++ b/internal/token.go
@@ -170,10 +170,6 @@ func providerAuthHeaderWorks(tokenURL string) bool {
 }
 
 func RetrieveToken(ctx context.Context, clientID, clientSecret, tokenURL string, v url.Values) (*Token, error) {
-	hc, err := ContextClient(ctx)
-	if err != nil {
-		return nil, err
-	}
 	bustedAuth := !providerAuthHeaderWorks(tokenURL)
 	if bustedAuth {
 		if clientID != "" {
@@ -191,7 +187,7 @@ func RetrieveToken(ctx context.Context, clientID, clientSecret, tokenURL string,
 	if !bustedAuth {
 		req.SetBasicAuth(url.QueryEscape(clientID), url.QueryEscape(clientSecret))
 	}
-	r, err := ctxhttp.Do(ctx, hc, req)
+	r, err := ctxhttp.Do(ctx, ContextClient(ctx), req)
 	if err != nil {
 		return nil, err
 	}
diff --git a/internal/transport.go b/internal/transport.go
index 783bd98..d16f9ae 100644
--- a/internal/transport.go
+++ b/internal/transport.go
@@ -19,50 +19,16 @@ var HTTPClient ContextKey
 // because nobody else can create a ContextKey, being unexported.
 type ContextKey struct{}
 
-// ContextClientFunc is a func which tries to return an *http.Client
-// given a Context value. If it returns an error, the search stops
-// with that error.  If it returns (nil, nil), the search continues
-// down the list of registered funcs.
-type ContextClientFunc func(context.Context) (*http.Client, error)
+var appengineClientHook func(context.Context) *http.Client
 
-var contextClientFuncs []ContextClientFunc
-
-func RegisterContextClientFunc(fn ContextClientFunc) {
-	contextClientFuncs = append(contextClientFuncs, fn)
-}
-
-func ContextClient(ctx context.Context) (*http.Client, error) {
+func ContextClient(ctx context.Context) *http.Client {
 	if ctx != nil {
 		if hc, ok := ctx.Value(HTTPClient).(*http.Client); ok {
-			return hc, nil
+			return hc
 		}
 	}
-	for _, fn := range contextClientFuncs {
-		c, err := fn(ctx)
-		if err != nil {
-			return nil, err
-		}
-		if c != nil {
-			return c, nil
-		}
+	if appengineClientHook != nil {
+		return appengineClientHook(ctx)
 	}
-	return http.DefaultClient, nil
-}
-
-func ContextTransport(ctx context.Context) http.RoundTripper {
-	hc, err := ContextClient(ctx)
-	// This is a rare error case (somebody using nil on App Engine).
-	if err != nil {
-		return ErrorTransport{err}
-	}
-	return hc.Transport
-}
-
-// ErrorTransport returns the specified error on RoundTrip.
-// This RoundTripper should be used in rare error cases where
-// error handling can be postponed to response handling time.
-type ErrorTransport struct{ Err error }
-
-func (t ErrorTransport) RoundTrip(*http.Request) (*http.Response, error) {
-	return nil, t.Err
+	return http.DefaultClient
 }
diff --git a/internal/transport_test.go b/internal/transport_test.go
deleted file mode 100644
index 8772ec5..0000000
--- a/internal/transport_test.go
+++ /dev/null
@@ -1,38 +0,0 @@
-// Copyright 2015 The Go 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 internal
-
-import (
-	"net/http"
-	"testing"
-
-	"golang.org/x/net/context"
-)
-
-func TestContextClient(t *testing.T) {
-	rc := &http.Client{}
-	RegisterContextClientFunc(func(context.Context) (*http.Client, error) {
-		return rc, nil
-	})
-
-	c := &http.Client{}
-	ctx := context.WithValue(context.Background(), HTTPClient, c)
-
-	hc, err := ContextClient(ctx)
-	if err != nil {
-		t.Fatalf("want valid client; got err = %v", err)
-	}
-	if hc != c {
-		t.Fatalf("want context client = %p; got = %p", c, hc)
-	}
-
-	hc, err = ContextClient(context.TODO())
-	if err != nil {
-		t.Fatalf("want valid client; got err = %v", err)
-	}
-	if hc != rc {
-		t.Fatalf("want registered client = %p; got = %p", c, hc)
-	}
-}
diff --git a/oauth2.go b/oauth2.go
index 6266950..7234e00 100644
--- a/oauth2.go
+++ b/oauth2.go
@@ -313,15 +313,11 @@ var HTTPClient internal.ContextKey
 // packages.
 func NewClient(ctx context.Context, src TokenSource) *http.Client {
 	if src == nil {
-		c, err := internal.ContextClient(ctx)
-		if err != nil {
-			return &http.Client{Transport: internal.ErrorTransport{Err: err}}
-		}
-		return c
+		return internal.ContextClient(ctx)
 	}
 	return &http.Client{
 		Transport: &Transport{
-			Base:   internal.ContextTransport(ctx),
+			Base:   internal.ContextClient(ctx).Transport,
 			Source: ReuseTokenSource(nil, src),
 		},
 	}