Add runtime.GOOS

src/runtime/os_js.go

@@ -0,0 +1,5 @@
+// +build js
+
+package runtime
+
+const GOOS = "js"

src/runtime/os_linux.go

@@ -0,0 +1,5 @@
+// +build linux
+
+package runtime
+
+const GOOS = "linux"