add blacklist syntax feature checking to 6to5/register

lib/6to5/register.js

@@ -20,6 +20,22 @@ sourceMapSupport.install({
 
 //
 
+var blacklist  = [];
+
+var blacklistTest = function (transformer, code) {
+  try {
+    eval(code);
+    blacklist.push(transformer);
+  } catch (err) {
+    if (err.name !== "SyntaxError") throw err;
+  }
+};
+
+blacklistTest("generators", "function test *() {}");
+blacklistTest("letScoping", "let test = 0;");
+
+//
+
 var ignoreRegex = /node_modules/;
 var exts        = {};
 var maps        = {};
@@ -31,6 +47,7 @@ var loader = function (m, filename) {
   }
 
   var result = to5.transformFileSync(filename, {
+    blacklist: blacklist,
     sourceMap: true
   });