google/appengine: change managed VMs context to context.Context

This follows the change to the google.golang.org/appengine packages
to use the standard context.Context type throughout.

Change-Id: I91b456105ee29f2088c3646145ea0a940f8ffaf4
Reviewed-on: https://go-review.googlesource.com/3172
Reviewed-by: Burcu Dogan <jbd@google.com>
Reviewed-by: David Symonds <dsymonds@golang.org>

google/source_appengine.go

@@ -21,7 +21,7 @@ var (
 	aeTokens = make(map[string]*tokenLock) // key is '\0'-separated scopes
 )
 
-var errInvalidContext = errors.New("oauth2: a valid appengine.Context is required")
+var errInvalidContext = errors.New("oauth2: context must come from appengine.NewContext")
 
 type tokenLock struct {
 	mu sync.Mutex // guards t; held while updating t