playground: add support for syscall/js package

Also modify the default "hello world" snippet to use it,
since it's a more general API.

Modernize update.sh slightly to handle variability in
environments better.

Updates #69

playground/playground.go

@@ -54,7 +54,7 @@ func main() {
 				})
 			}()
 		} else {
-			scope.Set("code", "package main\n\nimport (\n\t\"fmt\"\n\n\t\"github.com/gopherjs/gopherjs/js\"\n)\n\nfunc main() {\n\tfmt.Println(\"Hello, playground\")\n\tjs.Global.Call(\"alert\", \"Hello, JavaScript\")\n\tprintln(\"Hello, JS console\")\n}\n")
+			scope.Set("code", "package main\n\nimport (\n\t\"fmt\"\n\t\"syscall/js\"\n)\n\nfunc main() {\n\tfmt.Println(\"Hello, playground\")\n\tjs.Global().Call(\"alert\", \"Hello, JavaScript\")\n\tprintln(\"Hello, JS console\")\n}\n")
 			close(codeReady)
 		}
 		scope.Set("imports", true)

playground/update.sh

@@ -10,6 +10,17 @@ cleanup() {
 
 trap cleanup EXIT HUP INT TERM
 
+# This script relies on GOPATH mode. The GOPATH workspace
+# must contain both github.com/gopherjs/gopherjs and
+# github.com/gopherjs/gopherjs.github.io repositories.
+export GO111MODULE=off
+
+# TODO: This script relies on $(go env GOROOT) not being user-writable.
+# It can be improved to work even when $(go env GOROOT) is user-writable
+# by making a GOROOT copy here, immediately chmod-ing it to be non-user-writable,
+# and later on chmod-ing it be user-writable again.
+# See https://github.com/gopherjs/gopherjs.github.io/issues/69.
+
 go install github.com/gopherjs/gopherjs/...
 
 go generate github.com/gopherjs/gopherjs.github.io/playground/internal/imports
@@ -33,10 +44,12 @@ mkdir -p pkg/github.com/gopherjs/gopherjs
 cp "$GOPATH"/pkg/*_js_min/github.com/gopherjs/gopherjs/js.a pkg/github.com/gopherjs/gopherjs/js.a
 cp "$GOPATH"/pkg/*_js_min/github.com/gopherjs/gopherjs/nosync.a pkg/github.com/gopherjs/gopherjs/nosync.a
 
-# Make a copy of GOROOT that is user-writeable,
+# Make a copy of GOROOT that is user-writable,
 # use it to build and copy out standard library packages.
+echo "copying GOROOT from $(go env GOROOT) to $tmp/goroot"
 cp -a "$(go env GOROOT)" "$tmp/goroot"
 export GOROOT="$tmp/goroot"
+unset GOPHERJS_GOROOT  # force $GOROOT to be used
 gopherjs install -m \
          archive/tar \
          archive/zip \
@@ -125,6 +138,7 @@ gopherjs install -m \
          strconv \
          strings \
          sync/atomic \
+         syscall/js \
          testing \
          testing/iotest \
          testing/quick \