os: Drop support for go 1.15

1.15 specific files deleted.
1.16 specific files folded carefully into generic files, with goal of reducing diff with upstream.
Follows upstream 1.16 in making PathError etc. be aliases for the same errors in io/fs.

This fixes #2817 and lets us add io/ioutil to "make test-tinygo" on linux and mac.

src/os/os_anyos_test.go

@@ -4,6 +4,7 @@
 package os_test
 
 import (
+	"io/fs"
 	"io/ioutil"
 	. "os"
 	"path/filepath"
@@ -11,6 +12,7 @@ import (
 	"strconv"
 	"strings"
 	"testing"
+	"testing/fstest"
 	"time"
 )
 
@@ -55,8 +57,7 @@ func TestStatBadDir(t *testing.T) {
 	dir := TempDir()
 	badDir := filepath.Join(dir, "not-exist/really-not-exist")
 	_, err := Stat(badDir)
-	// TODO: PathError moved to io/fs in go 1.16; fix next line once we drop go 1.15 support.
-	if pe, ok := err.(*PathError); !ok || !IsNotExist(err) || pe.Path != badDir {
+	if pe, ok := err.(*fs.PathError); !ok || !IsNotExist(err) || pe.Path != badDir {
 		t.Errorf("Mkdir error = %#v; want PathError for path %q satisifying IsNotExist", err, badDir)
 	}
 }
@@ -123,8 +124,7 @@ func TestRemove(t *testing.T) {
 	if err == nil {
 		t.Errorf("TestRemove: remove of nonexistent file did not fail")
 	} else {
-		// FIXME: once we drop go 1.15, switch this to fs.PathError
-		if pe, ok := err.(*PathError); !ok {
+		if pe, ok := err.(*fs.PathError); !ok {
 			t.Errorf("TestRemove: expected PathError, got err %q", err.Error())
 		} else {
 			if pe.Path != f {
@@ -271,3 +271,55 @@ func TestUserHomeDir(t *testing.T) {
 		t.Fatalf("dir %s is not directory; type = %v", dir, fi.Mode())
 	}
 }
+
+func TestDirFS(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Log("TODO: implement Readdir for Windows")
+		return
+	}
+	if err := fstest.TestFS(DirFS("./testdata/dirfs"), "a", "b", "dir/x"); err != nil {
+		t.Fatal(err)
+	}
+
+	// Test that Open does not accept backslash as separator.
+	d := DirFS(".")
+	_, err := d.Open(`testdata\dirfs`)
+	if err == nil {
+		t.Fatalf(`Open testdata\dirfs succeeded`)
+	}
+}
+
+func TestDirFSPathsValid(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Log("skipping on Windows")
+		return
+	}
+
+	// TODO: switch back to t.TempDir once it's implemented
+	d, err := MkdirTemp("", "TestDirFSPathsValid")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer Remove(d)
+	if err := WriteFile(filepath.Join(d, "control.txt"), []byte(string("Hello, world!")), 0644); err != nil {
+		t.Fatal(err)
+	}
+	defer Remove(filepath.Join(d, "control.txt"))
+	if err := WriteFile(filepath.Join(d, `e:xperi\ment.txt`), []byte(string("Hello, colon and backslash!")), 0644); err != nil {
+		t.Fatal(err)
+	}
+	defer Remove(filepath.Join(d, `e:xperi\ment.txt`))
+
+	fsys := DirFS(d)
+	err = fs.WalkDir(fsys, ".", func(path string, e fs.DirEntry, err error) error {
+		if fs.ValidPath(e.Name()) {
+			t.Logf("%q ok", e.Name())
+		} else {
+			t.Errorf("%q INVALID", e.Name())
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+}