Introduce new compress option unsafe_arrows (#2278)

* Not always safe to convert a function expression to an arrow
  function when code depends on the function prototype existing.

Fixes #2271

lib/compress.js

@@ -84,6 +84,7 @@ function Compressor(options, false_by_default) {
         toplevel      : !!(options && options["top_retain"]),
         typeofs       : !false_by_default,
         unsafe        : false,
+        unsafe_arrows : false,
         unsafe_comps  : false,
         unsafe_Func   : false,
         unsafe_math   : false,
@@ -4748,7 +4749,7 @@ merge(Compressor.prototype, {
 
     OPT(AST_Function, function(self, compressor){
         tighten_body(self.body, compressor);
-        if (compressor.option("arrows")
+        if (compressor.option("unsafe_arrows")
             && compressor.option("ecma") >= 6
             && !self.name
             && !self.is_generator