Changing some nits.

Change-Id: I74140daa4acf876388f4f8ef83c475c1ae7d1865

google/internal/externalaccount/basecredentials.go

@@ -31,25 +31,20 @@ type Config struct {
 
 // TokenSource Returns an external account TokenSource struct. This is to be called by package google to construct a google.Credentials.
 func (c *Config) TokenSource(ctx context.Context) oauth2.TokenSource {
-	if c.ServiceAccountImpersonationURL == "" {
-		ts := tokenSource{
-			ctx:  ctx,
-			conf: c,
-		}
-		return oauth2.ReuseTokenSource(nil, ts)
-	}
-	imp := impersonateTokenSource{
-		ctx:    ctx,
-		url:    c.ServiceAccountImpersonationURL,
-		scopes: c.Scopes,
-	}
 	ts := tokenSource{
 		ctx:  ctx,
 		conf: c,
 	}
-	ts.conf.ServiceAccountImpersonationURL = ""
+	if c.ServiceAccountImpersonationURL == "" {
+		return oauth2.ReuseTokenSource(nil, ts)
+	}
 	ts.conf.Scopes = []string{"https://www.googleapis.com/auth/cloud-platform"}
-	imp.ts = oauth2.ReuseTokenSource(nil, ts)
+	imp := impersonateTokenSource{
+		ctx:    ctx,
+		url:    c.ServiceAccountImpersonationURL,
+		scopes: c.Scopes,
+		ts: oauth2.ReuseTokenSource(nil, ts),
+	}
 	return oauth2.ReuseTokenSource(nil, imp)
 }
 

google/internal/externalaccount/impersonate.go

@@ -36,7 +36,7 @@ type impersonateTokenSource struct {
 	scopes []string
 }
 
-// impersonate performs the exchange to get a temporary service account
+// Token performs the exchange to get a temporary service account
 func (its impersonateTokenSource) Token() (*oauth2.Token, error) {
 	reqBody := generateAccessTokenReq{
 		Lifetime: "3600s",