When unsafe is set, evaluate [...].join() if possible

Close #298

lib/compress.js

@@ -746,6 +746,19 @@ merge(Compressor.prototype, {
             if (d && d.constant && d.init) return ev(d.init, compressor);
             throw def;
         });
+        def(AST_Call, function(compressor){
+            if (compressor.option("unsafe")) {
+                if (this.expression instanceof AST_Dot
+                    && this.expression.expression instanceof AST_Array
+                    && this.expression.property == "join") {
+                    var x =  this.expression.expression.elements.map(function(el){
+                        return ev(el, compressor);
+                    });
+                    return x.join(ev(this.args[0]));
+                }
+            }
+            throw def;
+        });
     })(function(node, func){
         node.DEFMETHOD("_eval", func);
     });
@@ -1704,7 +1717,7 @@ merge(Compressor.prototype, {
                 return make_node(AST_Undefined, self).transform(compressor);
             }
         }
-        return self;
+        return self.evaluate(compressor)[0];
     });
 
     OPT(AST_New, function(self, compressor){