[es8] Throw SyntaxError when tail call expressions occur in non-strict mode.

test/message/syntactic-tail-call-sloppy.js

Index: test/message/syntactic-tail-call-sloppy.js
diff --git a/test/message/syntactic-tail-call-in-extends.js b/test/message/syntactic-tail-call-sloppy.js
similarity index 84%
copy from test/message/syntactic-tail-call-in-extends.js
copy to test/message/syntactic-tail-call-sloppy.js
index 02b578d4ca1a301ea7f39e814ea0d25924c16224..3973fc6d182643802701edc539b64eed45e16af3 100644
--- a/test/message/syntactic-tail-call-in-extends.js
+++ b/test/message/syntactic-tail-call-sloppy.js
@@ -5,5 +5,5 @@
 // Flags: --harmony-explicit-tailcalls
 
 function g() {
-  return class A extends continue f() {};
+  return  continue  f()  ;
 }