enable unsafe for test/ufuzz.js (#2819)

- introduce `unsafe_undefined`
- safer `.toString()` compression

Miscellaneous
- rename `unsafe_Function`

lib/compress.js

@@ -84,10 +84,11 @@ function Compressor(options, false_by_default) {
         typeofs       : !false_by_default,
         unsafe        : false,
         unsafe_comps  : false,
-        unsafe_Func   : false,
+        unsafe_Function: false,
         unsafe_math   : false,
         unsafe_proto  : false,
         unsafe_regexp : false,
+        unsafe_undefined: false,
         unused        : !false_by_default,
         warnings      : false,
     }, true);
@@ -4100,11 +4101,13 @@ merge(Compressor.prototype, {
                 break;
             } else if (exp instanceof AST_Dot) switch(exp.property) {
               case "toString":
-                if (self.args.length == 0) return make_node(AST_Binary, self, {
-                    left: make_node(AST_String, self, { value: "" }),
-                    operator: "+",
-                    right: exp.expression
-                }).optimize(compressor);
+                if (self.args.length == 0 && !exp.expression.may_throw_on_access(compressor)) {
+                    return make_node(AST_Binary, self, {
+                        left: make_node(AST_String, self, { value: "" }),
+                        operator: "+",
+                        right: exp.expression
+                    }).optimize(compressor);
+                }
                 break;
               case "join":
                 if (exp.expression instanceof AST_Array) EXIT: {
@@ -4212,7 +4215,7 @@ merge(Compressor.prototype, {
                 break;
             }
         }
-        if (compressor.option("unsafe_Func")
+        if (compressor.option("unsafe_Function")
             && is_undeclared_ref(exp)
             && exp.name == "Function") {
             // new Function() => function(){}
@@ -5220,7 +5223,7 @@ merge(Compressor.prototype, {
     }
 
     OPT(AST_Undefined, function(self, compressor){
-        if (compressor.option("unsafe")) {
+        if (compressor.option("unsafe_undefined")) {
             var undef = find_variable(compressor, "undefined");
             if (undef) {
                 var ref = make_node(AST_SymbolRef, self, {