Reland "godoc: skip build tag annotations when displaying examples"

After moving the filepath.Walk example to a standalone example file
in CL 122237 (so it could use a standalone function), godoc includes
the build tag annotation ("// +build !windows,!plan9" in this case)
in the runnable example.  The example runs correctly, but the
annotation might be confusing for new users.

I think godoc should skip these annotations when displaying examples.

Fixes golang/go#26490.

Skipped the testcase for now, since it triggers a false positive:
golang/go#26627

Change-Id: I1da4b3b7e1e5a85a76773e25d9355b3f92479c19

Author: mostynb

godoc/godoc.go

@@ -666,6 +666,7 @@ func (p *Presentation) example_htmlFunc(info *PageInfo, funcName string) string
 		play := ""
 		if eg.Play != nil && p.ShowPlayground {
 			var buf bytes.Buffer
+			eg.Play.Comments = filterOutBuildAnnotations(eg.Play.Comments)
 			if err := format.Node(&buf, info.FSet, eg.Play); err != nil {
 				log.Print(err)
 			} else {
@@ -694,6 +695,23 @@ func (p *Presentation) example_htmlFunc(info *PageInfo, funcName string) string
 	return buf.String()
 }
 
+func filterOutBuildAnnotations(cg []*ast.CommentGroup) []*ast.CommentGroup {
+	if len(cg) == 0 {
+		return cg
+	}
+
+	for i := range cg {
+		if !strings.HasPrefix(cg[i].Text(), "+build ") {
+			// Found the first non-build tag, return from here until the end
+			// of the slice.
+			return cg[i:]
+		}
+	}
+
+	// There weren't any non-build tags, return an empty slice.
+	return []*ast.CommentGroup{}
+}
+
 // example_nameFunc takes an example function name and returns its display
 // name. For example, "Foo_Bar_quux" becomes "Foo.Bar (Quux)".
 func (p *Presentation) example_nameFunc(s string) string {