html/template: add DefinedTemplates to html/template

It is not important to add, since it's only used for creating an error message,
but for consistency in the API between text/template and html/template
it should be provided here.

The implementation just calls the one in text/template.

Fixes #13349.

Change-Id: I0882849e06a58f1e38b00eb89d79ac39777309b2
Reviewed-on: https://go-review.googlesource.com/17172Reviewed-by: Andrew Gerrand <adg@golang.org>

src/html/template/template.go

@@ -80,7 +80,7 @@ func (t *Template) escape() error {
 	defer t.nameSpace.mu.Unlock()
 	if t.escapeErr == nil {
 		if t.Tree == nil {
-			return fmt.Errorf("template: %q is an incomplete or empty template%s", t.Name(), t.text.DefinedTemplates())
+			return fmt.Errorf("template: %q is an incomplete or empty template%s", t.Name(), t.DefinedTemplates())
 		}
 		if err := escapeTemplate(t, t.text.Root, t.Name()); err != nil {
 			return err
@@ -143,6 +143,13 @@ func (t *Template) lookupAndEscapeTemplate(name string) (tmpl *Template, err err
 	return tmpl, err
 }
 
+// DefinedTemplates returns a string listing the defined templates,
+// prefixed by the string "defined templates are: ". If there are none,
+// it returns the empty string. Used to generate an error message.
+func (t *Template) DefinedTemplates() string {
+	return t.text.DefinedTemplates()
+}
+
 // Parse parses a string into a template. Nested template definitions
 // will be associated with the top-level template t. Parse may be
 // called multiple times to parse definitions of templates to associate