time: return correct zone info on js/wasm

Fixes #28265

Change-Id: I0a13e9f9c216647e42127a59a80b0f19618169c1
Reviewed-on: https://go-review.googlesource.com/c/143577
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Richard Musiol <neelance@gmail.com>

src/go/build/deps_test.go

@@ -159,6 +159,7 @@ var pkgDeps = map[string][]string{
 		// Other time dependencies:
 		"internal/syscall/windows/registry",
 		"syscall",
+		"syscall/js",
 	},
 
 	"internal/poll":    {"L0", "internal/race", "syscall", "time", "unicode/utf16", "unicode/utf8", "internal/syscall/windows"},

src/time/zoneinfo_js.go

@@ -0,0 +1,66 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build js,wasm
+
+package time
+
+import (
+	"runtime"
+	"syscall/js"
+)
+
+var zoneSources = []string{
+	"/usr/share/zoneinfo/",
+	"/usr/share/lib/zoneinfo/",
+	"/usr/lib/locale/TZ/",
+	runtime.GOROOT() + "/lib/time/zoneinfo.zip",
+}
+
+func initLocal() {
+	localLoc.name = "Local"
+
+	z := zone{}
+	d := js.Global().Get("Date").New()
+	offset := d.Call("getTimezoneOffset").Int() * -1
+	z.offset = offset * 60
+	// According to https://tc39.github.io/ecma262/#sec-timezoneestring,
+	// the timezone name from (new Date()).toTimeString() is an implementation-dependent
+	// result, and in Google Chrome, it gives the fully expanded name rather than
+	// the abbreviation.
+	// Hence, we construct the name from the offset.
+	z.name = "UTC"
+	if offset < 0 {
+		z.name += "-"
+	} else {
+		z.name += "+"
+	}
+	z.name += itoa(offset / 60)
+	min := offset % 60
+	if min != 0 {
+		z.name += ":" + itoa(min)
+	}
+	localLoc.zone = []zone{z}
+}
+
+// itoa is just the fast path implementation copied from strconv.Itoa.
+// No timezone hour can exceed 100, so the fast path will always satisfy.
+func itoa(i int) string {
+	if i < 10 {
+		return digits[i : i+1]
+	}
+	return smallsString[i*2 : i*2+2]
+}
+
+const smallsString = "00010203040506070809" +
+	"10111213141516171819" +
+	"20212223242526272829" +
+	"30313233343536373839" +
+	"40414243444546474849" +
+	"50515253545556575859" +
+	"60616263646566676869" +
+	"70717273747576777879" +
+	"80818283848586878889" +
+	"90919293949596979899"
+const digits = "0123456789"

src/time/zoneinfo_unix.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build aix darwin,386 darwin,amd64 dragonfly freebsd js,wasm linux,!android nacl netbsd openbsd solaris
+// +build aix darwin,386 darwin,amd64 dragonfly freebsd linux,!android nacl netbsd openbsd solaris
 
 // Parse "zoneinfo" time zone file.
 // This is a fairly standard file format used on OS X, Linux, BSD, Sun, and others.