td, err := os.Open("testdata/src/pkg")
	if err != nil {
		t.Fatal(err)
	}
	fis, err := td.Readdir(0)
	if err != nil {
		t.Fatal(err)
	}
	for _, fi := range fis {
		if !fi.IsDir() {
			continue
		}

		// TODO(gri) remove extra pkg directory eventually
		goldenFile := filepath.Join("testdata", "src", "pkg", fi.Name(), "golden.txt")
		w := NewWalker(nil, "testdata/src/pkg")
		pkg, _ := w.import_(fi.Name())

	{"x+24(FP)", "x+24(FP)"},
	{"x·y(SB)", "x.y(SB)"},
	{"x·y(SP)", "x.y(SP)"},
	{"x·y+8(SB)", "x.y+8(SB)"},
	{"x·y+8(SP)", "x.y+8(SP)"},
	{"y+56(FP)", "y+56(FP)"},
	{"·AddUint32(SB)", "pkg.AddUint32(SB)"},
	{"·callReflect(SB)", "pkg.callReflect(SB)"},
	{"[X0-X0]", "[X0-X0]"},
	{"[ Z9 - Z12 ]", "[Z9-Z12]"},
	{"[X0-AX]", "[X0-AX]"},
	{"[AX-X0]", "[AX-X0]"},

effect to systems satisfying one of the constraints
(see https://golang.org/pkg/go/build/#hdr-Build_Constraints for details about the constraint syntax).
For example:

	// #cgo CFLAGS: -DPNG_DEBUG=1
	// #cgo amd64 386 CFLAGS: -DX86=1
	// #cgo LDFLAGS: -lpng
	// #include <png.h>
	import "C"

Alternatively, CPPFLAGS and LDFLAGS may be obtained via the pkg-config tool
using a '#cgo pkg-config:' directive followed by the package names.
For example:

			//   imported anyway.
			if ip := pkg.ImportPath; pkg.Standard && ip != "unsafe" && !strings.HasPrefix(ip, "vendor/") && !internalPkg.MatchString(ip) {
				stdPackages = append(stdPackages, ip)
			}
			importDir[pkg.ImportPath] = pkg.Dir
			if len(pkg.ImportMap) > 0 {
				importMap[pkg.Dir] = make(map[string]string, len(pkg.ImportMap))
			}
			for k, v := range pkg.ImportMap {
				importMap[pkg.Dir][k] = v
			}

		return err
	}

	// Copy only the relevant subdirectories from pkg: pkg/include and the
	// platform-native binaries in pkg/tool.
	if err := adb("exec-out", "mkdir", "-p", path.Join(deviceGoroot, "pkg", "tool")); err != nil {
		return err
	}
	if err := adb("push", filepath.Join(goroot, "pkg", "include"), path.Join(deviceGoroot, "pkg", "include")); err != nil {
		return err
	}

	//
	// User-defined records should have keys of the following form:
	//	VENDOR.keyword
	// Where VENDOR is some namespace in all uppercase, and keyword may
	// not contain the '=' character (e.g., "GOLANG.pkg.version").
	// The key and value should be non-empty UTF-8 strings.
	//
	// When Writer.WriteHeader is called, PAX records derived from the
	// other fields in Header take precedence over PAXRecords.

Flags:

	-D name[=value]
		Predefine symbol name with an optional simple value.
		Can be repeated to define multiple symbols.
	-I dir1 -I dir2
		Search for #include files in dir1, dir2, etc,
		after consulting $GOROOT/pkg/$GOOS_$GOARCH.
	-S
		Print assembly and machine code.
	-V
		Print assembler version and exit.
	-debug
		Dump instructions as they are parsed.
	-dynlink

		headers: []*Header{{
			Typeflag: TypeReg,
			Name:     "file",
			Uname:    strings.Repeat("long", 10),
			ModTime:  time.Unix(0, 0),
			PAXRecords: map[string]string{
				"GOLANG.pkg": "tar",
				"comment":    "Hello, 世界",
				"uname":      strings.Repeat("long", 10),
			},
			Format: FormatPAX,
		}},
	}, {
		file: "testdata/pax-global-records.tar",
		headers: []*Header{{

		//
		// Typical apps have the zoneinfo.zip in the root of their app bundle,
		// read by the time package as the working directory at initialization.
		// As we move the working directory to the GOROOT pkg directory, we
		// install the zoneinfo.zip file in the pkgpath.
		err := cp(
			filepath.Join(dstbase, pkgpath),
			filepath.Join(cwd, "lib", "time", "zoneinfo.zip"),
		)
		if err != nil {
			return "", err

				PAXRecords: map[string]string{
					"path":           "FILE", // Should be ignored
					"GNU.sparse.map": "0,0",  // Should be ignored
					"comment":        "Hello, 世界",
					"GOLANG.pkg":     "tar",
				},
			}, nil},
			testClose{nil},
		},
	}, {
		// Craft a theoretically valid PAX archive with global headers.
		// The GNU and BSD tar tools do not parse these the same way.
		//