gc: diagnose \ in import path

R=ken2
CC=golang-dev
https://golang.org/cl/5609044

src/cmd/gc/lex.c

@@ -574,6 +574,11 @@ importfile(Val *f, int line)
 		yyerror("import path contains NUL");
 		errorexit();
 	}
+	
+	if(strchr(f->u.sval->s, '\\')) {
+		yyerror("import path contains backslash; use slash");
+		errorexit();
+	}
 
 	// The package name main is no longer reserved,
 	// but we reserve the import path "main" to identify

test/import5.go

@@ -0,0 +1,11 @@
+// errchk $G -e $D/$F.go
+
+// Copyright 2009 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.
+
+// import paths are slash-separated; reject backslash
+
+package main
+
+import `net\http`  // ERROR "backslash"