fix corner case in unsafe (#3646)

lib/compress.js

@@ -2872,9 +2872,11 @@ merge(Compressor.prototype, {
     }
 
     function convert_to_predicate(obj) {
+        var map = Object.create(null);
         Object.keys(obj).forEach(function(key) {
-            obj[key] = makePredicate(obj[key]);
+            map[key] = makePredicate(obj[key]);
         });
+        return map;
     }
 
     AST_Lambda.DEFMETHOD("first_statement", function() {
@@ -2897,7 +2899,7 @@ merge(Compressor.prototype, {
         "toString",
         "valueOf",
     ];
-    var native_fns = {
+    var native_fns = convert_to_predicate({
         Array: [
             "indexOf",
             "join",
@@ -2933,9 +2935,8 @@ merge(Compressor.prototype, {
             "toUpperCase",
             "trim",
         ].concat(object_fns),
-    };
-    convert_to_predicate(native_fns);
-    var static_fns = {
+    });
+    var static_fns = convert_to_predicate({
         Array: [
             "isArray",
         ],
@@ -2975,8 +2976,7 @@ merge(Compressor.prototype, {
         String: [
             "fromCharCode",
         ],
-    };
-    convert_to_predicate(static_fns);
+    });
 
     // methods to evaluate a constant expression
     (function(def) {
@@ -3199,7 +3199,7 @@ merge(Compressor.prototype, {
             Object: Object,
             String: String,
         };
-        var static_values = {
+        var static_values = convert_to_predicate({
             Math: [
                 "E",
                 "LN10",
@@ -3217,8 +3217,7 @@ merge(Compressor.prototype, {
                 "NEGATIVE_INFINITY",
                 "POSITIVE_INFINITY",
             ],
-        };
-        convert_to_predicate(static_values);
+        });
         var regexp_props = makePredicate("global ignoreCase multiline source");
         def(AST_PropAccess, function(compressor, cached, depth) {
             if (compressor.option("unsafe")) {