exp/template/html: do not depend on reflection on internal fields

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5286049

src/pkg/exp/template/html/context.go

@@ -25,6 +25,10 @@ type context struct {
 	err     *Error
 }
 
+func (c context) String() string {
+	return fmt.Sprintf("{%v %v %v %v %v %v %v}", c.state, c.delim, c.urlPart, c.jsCtx, c.attr, c.element, c.err)
+}
+
 // eq returns whether two contexts are equal.
 func (c context) eq(d context) bool {
 	return c.state == d.state &&