undo CL 5330066 / 6a5647d82728

I promised it wouldn't last very long.
People who really need this can sync to 6a5647d82728.

««« original CL description
gc: add GOEXPERIMENT=os.Error

This won't last long, I promise.

R=ken2
CC=golang-dev
https://golang.org/cl/5330066
»»»

R=ken2
CC=golang-dev
https://golang.org/cl/5333053

src/cmd/gc/go.h

@@ -852,7 +852,6 @@ EXTERN	int	typecheckok;
 EXTERN	int	compiling_runtime;
 
 EXTERN	int	rune32;
-EXTERN	int	oserror;
 
 /*
  *	y.tab.c

src/cmd/gc/lex.c

@@ -38,7 +38,6 @@ static struct {
 	int *val;
 } exper[] = {
 	{"rune32", &rune32},
-	{"os.Error", &oserror},
 };
 
 static void

src/cmd/gc/subr.c

@@ -2967,17 +2967,6 @@ mkpkg(Strlit *path)
 	p->prefix = pathtoprefix(path->s);
 	p->link = phash[h];
 	phash[h] = p;
-	
-	// If the compiler was built with
-	//	GOEXPERIMENT=os.Error
-	// define os.Error as an alias for error.
-	// Terrible and won't last long, but useful for transitions.
-	if(oserror && strcmp(path->s, "os") == 0) {
-		Sym *s;
-		s = pkglookup("Error", p);
-		s->def = typenod(errortype);
-	}
-
 	return p;
 }