First wave of review updates.

Change-Id: Ibfe8cb23f12c516d9264fcbbee8d8af64b458c89

google/internal/externalaccount/urlcredsource.go

@@ -5,39 +5,39 @@
 package externalaccount
 
 import (
+	"context"
 	"encoding/json"
 	"errors"
 	"fmt"
+	"golang.org/x/oauth2"
 	"io"
 	"io/ioutil"
 	"net/http"
-	"strings"
 )
 
 type urlCredentialSource struct {
 	URL     string
 	Headers map[string]string
 	Format  format
+	ctx     context.Context
 }
 
 func (cs urlCredentialSource) subjectToken() (string, error) {
-	client := http.Client{}
-	req, err := http.NewRequest("GET", cs.URL, strings.NewReader(""))
+	client := oauth2.NewClient(cs.ctx, nil)
+	req, err := http.NewRequest("GET", cs.URL, nil)
 
 	for key, val := range cs.Headers {
 		req.Header.Add(key, val)
 	}
 	resp, err := client.Do(req)
 	if err != nil {
-		fmt.Errorf("oauth2/google: invalid response when retrieving subject token: %v", err)
-		return "", err
+		return "", fmt.Errorf("oauth2/google: invalid response when retrieving subject token: %v", err)
 	}
 	defer resp.Body.Close()
 
 	tokenBytes, err := ioutil.ReadAll(io.LimitReader(resp.Body, 1<<20))
 	if err != nil {
-		fmt.Errorf("oauth2/google: invalid body in subject token URL query: %v", err)
-		return "", err
+		return "", fmt.Errorf("oauth2/google: invalid body in subject token URL query: %v", err)
 	}
 
 	switch cs.Format.Type {