compiler/natives: use embed package

It's now possible to use the new embed package in the standard library
to simplify the implementation of natives.FS and reduce its dependence
on packages outside the standard library.

This also drops support for the gopherjsdev tag where the natives could
be read directly from disk without regenerating and rebuilding the
gopherjs binary. That mode is less helpful now that file changes can be
picked up just by rebuilding (without regenerating).
It also can't be made to work reliably in module mode since module mode
doesn't have the concept of a system-wide always-present workspace like
GOPATH mode had. One-off occasional needs can probably be better served
by temporary local code changes instead.

Author: dmitshur

build/context.go

@@ -60,7 +60,7 @@ func DefaultEnv() Env {
 	return e
 }
 
-// XContext is an extension of go/build.Context with GopherJS-specifc features.
+// XContext is an extension of go/build.Context with GopherJS-specific features.
 //
 // It abstracts away several different sources GopherJS can load its packages
 // from, with a minimal API.
@@ -313,7 +313,7 @@ func embeddedCtx(embedded http.FileSystem, e Env) *simpleCtx {
 // overlayCtx creates simpleCtx that imports from the embedded standard library
 // overlays.
 func overlayCtx(e Env) *simpleCtx {
-	return embeddedCtx(&withPrefix{fs: natives.FS, prefix: e.GOROOT}, e)
+	return embeddedCtx(&withPrefix{fs: http.FS(natives.FS), prefix: e.GOROOT}, e)
 }
 
 // gopherjsCtx creates a simpleCtx that imports from the embedded gopherjs