html: add NodeType.String() method

Uses stringer to add String method to html.NodeType.

Fixes golang/go#77006

Change-Id: I8120215f859f4c3e6f17757e106cff3936bfa1ba
GitHub-Last-Rev: 2082ce04b3
GitHub-Pull-Request: golang/net#244
Reviewed-on: https://go-review.googlesource.com/c/net/+/738100
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

html/node.go

@@ -11,6 +11,7 @@ import (
 // A NodeType is the type of a Node.
 type NodeType uint32
 
+//go:generate stringer -type NodeType
 const (
 	ErrorNode NodeType = iota
 	TextNode

html/nodetype_string.go

@@ -0,0 +1,31 @@
+// Code generated by "stringer -type NodeType"; DO NOT EDIT.
+
+package html
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[ErrorNode-0]
+	_ = x[TextNode-1]
+	_ = x[DocumentNode-2]
+	_ = x[ElementNode-3]
+	_ = x[CommentNode-4]
+	_ = x[DoctypeNode-5]
+	_ = x[RawNode-6]
+	_ = x[scopeMarkerNode-7]
+}
+
+const _NodeType_name = "ErrorNodeTextNodeDocumentNodeElementNodeCommentNodeDoctypeNodeRawNodescopeMarkerNode"
+
+var _NodeType_index = [...]uint8{0, 9, 17, 29, 40, 51, 62, 69, 84}
+
+func (i NodeType) String() string {
+	idx := int(i) - 0
+	if i < 0 || idx >= len(_NodeType_index)-1 {
+		return "NodeType(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _NodeType_name[_NodeType_index[idx]:_NodeType_index[idx+1]]
+}