template: added ParseFile method for template.Template

Fixes #971.

Parse/ParseFile methods of Template now match template.Parse and .ParseFile methods.
Also made tests being run on Parse and ParseFile be run on Template.ParseFile as well.

R=r
CC=golang-dev
https://golang.org/cl/1741059

src/pkg/template/template.go

@@ -888,6 +888,16 @@ func (t *Template) Parse(s string) (err os.Error) {
 	return nil
 }
 
+// ParseFile is like Parse but reads the template definition from the
+// named file.
+func (t *Template) ParseFile(filename string) (err os.Error) {
+	b, err := ioutil.ReadFile(filename)
+	if err != nil {
+		return err
+	}
+	return t.Parse(string(b))
+}
+
 // Execute applies a parsed template to the specified data object,
 // generating output to wr.
 func (t *Template) Execute(data interface{}, wr io.Writer) (err os.Error) {

src/pkg/template/template_test.go

@@ -398,9 +398,23 @@ func TestAll(t *testing.T) {
 	testAll(t, func(test *Test) (*Template, os.Error) { return Parse(test.in, formatters) })
 	// ParseFile
 	testAll(t, func(test *Test) (*Template, os.Error) {
-		ioutil.WriteFile("_test/test.tmpl", []byte(test.in), 0600)
+		err := ioutil.WriteFile("_test/test.tmpl", []byte(test.in), 0600)
+		if err != nil {
+			t.Error("unexpected write error:", err)
+			return nil, err
+		}
 		return ParseFile("_test/test.tmpl", formatters)
 	})
+	// tmpl.ParseFile
+	testAll(t, func(test *Test) (*Template, os.Error) {
+		err := ioutil.WriteFile("_test/test.tmpl", []byte(test.in), 0600)
+		if err != nil {
+			t.Error("unexpected write error:", err)
+			return nil, err
+		}
+		tmpl := New(formatters)
+		return tmpl, tmpl.ParseFile("_test/test.tmpl")
+	})
 }
 
 func testAll(t *testing.T, parseFunc func(*Test) (*Template, os.Error)) {