diff --git a/lib/compress.js b/lib/compress.js
index f7232e9a..3f2e1ec6 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -185,90 +185,89 @@ function Compressor(options, false_by_default) {
     };
 }
 
-Compressor.prototype = new TreeTransformer;
-merge(Compressor.prototype, {
-    option: function(key) { return this.options[key] },
-    exposed: function(def) {
-        if (def.exported) return true;
-        if (def.undeclared) return true;
-        if (!(def.global || def.scope.resolve() instanceof AST_Toplevel)) return false;
-        var toplevel = this.toplevel;
-        return !all(def.orig, function(sym) {
-            return toplevel[sym instanceof AST_SymbolDefun ? "funcs" : "vars"];
-        });
-    },
-    compress: function(node) {
-        node = node.resolve_defines(this);
-        node.hoist_exports(this);
-        if (this.option("expression")) {
-            node.process_expression(true);
-        }
-        var merge_vars = this.options.merge_vars;
-        var passes = +this.options.passes || 1;
-        var min_count = 1 / 0;
-        var stopping = false;
-        var mangle = { ie: this.option("ie") };
-        for (var pass = 0; pass < passes; pass++) {
-            node.figure_out_scope(mangle);
-            if (pass > 0 || this.option("reduce_vars"))
-                node.reset_opt_flags(this);
-            this.options.merge_vars = merge_vars && (stopping || pass == passes - 1);
-            node = node.transform(this);
-            if (passes > 1) {
-                var count = 0;
-                node.walk(new TreeWalker(function() {
-                    count++;
-                }));
-                AST_Node.info("pass {pass}: last_count: {min_count}, count: {count}", {
-                    pass: pass,
-                    min_count: min_count,
-                    count: count,
-                });
-                if (count < min_count) {
-                    min_count = count;
-                    stopping = false;
-                } else if (stopping) {
-                    break;
-                } else {
-                    stopping = true;
-                }
+Compressor.prototype = new TreeTransformer(function(node, descend, in_list) {
+    if (node._squeezed) return node;
+    var is_scope = node instanceof AST_Scope;
+    if (is_scope) {
+        node.hoist_properties(this);
+        node.hoist_declarations(this);
+        node.process_boolean_returns(this);
+    }
+    // Before https://github.com/mishoo/UglifyJS/pull/1602 AST_Node.optimize()
+    // would call AST_Node.transform() if a different instance of AST_Node is
+    // produced after OPT().
+    // This corrupts TreeWalker.stack, which cause AST look-ups to malfunction.
+    // Migrate and defer all children's AST_Node.transform() to below, which
+    // will now happen after this parent AST_Node has been properly substituted
+    // thus gives a consistent AST snapshot.
+    descend(node, this);
+    // Existing code relies on how AST_Node.optimize() worked, and omitting the
+    // following replacement call would result in degraded efficiency of both
+    // output and performance.
+    descend(node, this);
+    var opt = node.optimize(this);
+    if (is_scope && opt === node && !this.has_directive("use asm") && !opt.pinned()) {
+        opt.drop_unused(this);
+        if (opt.merge_variables(this)) opt.drop_unused(this);
+        descend(opt, this);
+    }
+    if (opt === node) opt._squeezed = true;
+    return opt;
+});
+Compressor.prototype.option = function(key) {
+    return this.options[key];
+};
+Compressor.prototype.exposed = function(def) {
+    if (def.exported) return true;
+    if (def.undeclared) return true;
+    if (!(def.global || def.scope.resolve() instanceof AST_Toplevel)) return false;
+    var toplevel = this.toplevel;
+    return !all(def.orig, function(sym) {
+        return toplevel[sym instanceof AST_SymbolDefun ? "funcs" : "vars"];
+    });
+};
+Compressor.prototype.compress = function(node) {
+    node = node.resolve_defines(this);
+    node.hoist_exports(this);
+    if (this.option("expression")) {
+        node.process_expression(true);
+    }
+    var merge_vars = this.options.merge_vars;
+    var passes = +this.options.passes || 1;
+    var min_count = 1 / 0;
+    var stopping = false;
+    var mangle = { ie: this.option("ie") };
+    for (var pass = 0; pass < passes; pass++) {
+        node.figure_out_scope(mangle);
+        if (pass > 0 || this.option("reduce_vars"))
+            node.reset_opt_flags(this);
+        this.options.merge_vars = merge_vars && (stopping || pass == passes - 1);
+        node = node.transform(this);
+        if (passes > 1) {
+            var count = 0;
+            node.walk(new TreeWalker(function() {
+                count++;
+            }));
+            AST_Node.info("pass {pass}: last_count: {min_count}, count: {count}", {
+                pass: pass,
+                min_count: min_count,
+                count: count,
+            });
+            if (count < min_count) {
+                min_count = count;
+                stopping = false;
+            } else if (stopping) {
+                break;
+            } else {
+                stopping = true;
             }
         }
-        if (this.option("expression")) {
-            node.process_expression(false);
-        }
-        return node;
-    },
-    before: function(node, descend, in_list) {
-        if (node._squeezed) return node;
-        var is_scope = node instanceof AST_Scope;
-        if (is_scope) {
-            node.hoist_properties(this);
-            node.hoist_declarations(this);
-            node.process_boolean_returns(this);
-        }
-        // Before https://github.com/mishoo/UglifyJS/pull/1602 AST_Node.optimize()
-        // would call AST_Node.transform() if a different instance of AST_Node is
-        // produced after OPT().
-        // This corrupts TreeWalker.stack, which cause AST look-ups to malfunction.
-        // Migrate and defer all children's AST_Node.transform() to below, which
-        // will now happen after this parent AST_Node has been properly substituted
-        // thus gives a consistent AST snapshot.
-        descend(node, this);
-        // Existing code relies on how AST_Node.optimize() worked, and omitting the
-        // following replacement call would result in degraded efficiency of both
-        // output and performance.
-        descend(node, this);
-        var opt = node.optimize(this);
-        if (is_scope && opt === node && !this.has_directive("use asm") && !opt.pinned()) {
-            opt.drop_unused(this);
-            if (opt.merge_variables(this)) opt.drop_unused(this);
-            descend(opt, this);
-        }
-        if (opt === node) opt._squeezed = true;
-        return opt;
     }
-});
+    if (this.option("expression")) {
+        node.process_expression(false);
+    }
+    return node;
+};
 
 (function(OPT) {
     OPT(AST_Node, function(self, compressor) {
diff --git a/lib/utils.js b/lib/utils.js
index 69c2dcd1..4c465844 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -96,15 +96,6 @@ function defaults(args, defs, croak) {
     return defs;
 }
 
-function merge(obj, ext) {
-    var count = 0;
-    for (var i in ext) if (HOP(ext, i)) {
-        obj[i] = ext[i];
-        count++;
-    }
-    return count;
-}
-
 function noop() {}
 function return_false() { return false; }
 function return_true() { return true; }
@@ -171,63 +162,80 @@ function all(array, predicate) {
 }
 
 function Dictionary() {
-    this._values = Object.create(null);
-    this._size = 0;
+    this.values = Object.create(null);
 }
 Dictionary.prototype = {
     set: function(key, val) {
-        if (!this.has(key)) ++this._size;
-        this._values["$" + key] = val;
+        if (key == "__proto__") {
+            this.proto_value = val;
+        } else {
+            this.values[key] = val;
+        }
         return this;
     },
     add: function(key, val) {
-        if (this.has(key)) {
-            this.get(key).push(val);
+        var list = this.get(key);
+        if (list) {
+            list.push(val);
         } else {
             this.set(key, [ val ]);
         }
         return this;
     },
-    get: function(key) { return this._values["$" + key] },
+    get: function(key) {
+        return key == "__proto__" ? this.proto_value : this.values[key];
+    },
     del: function(key) {
-        if (this.has(key)) {
-            --this._size;
-            delete this._values["$" + key];
+        if (key == "__proto__") {
+            delete this.proto_value;
+        } else {
+            delete this.values[key];
         }
         return this;
     },
-    has: function(key) { return ("$" + key) in this._values },
+    has: function(key) {
+        return key == "__proto__" ? "proto_value" in this : key in this.values;
+    },
     all: function(predicate) {
-        for (var i in this._values)
-            if (!predicate(this._values[i], i.substr(1)))
-                return false;
+        for (var i in this.values)
+            if (!predicate(this.values[i], i)) return false;
+        if ("proto_value" in this && !predicate(this.proto_value, "__proto__")) return false;
         return true;
     },
     each: function(f) {
-        for (var i in this._values)
-            f(this._values[i], i.substr(1));
+        for (var i in this.values)
+            f(this.values[i], i);
+        if ("proto_value" in this) f(this.proto_value, "__proto__");
     },
     size: function() {
-        return this._size;
+        return Object.keys(this.values).length + ("proto_value" in this);
     },
     map: function(f) {
         var ret = [];
-        for (var i in this._values)
-            ret.push(f(this._values[i], i.substr(1)));
+        for (var i in this.values)
+            ret.push(f(this.values[i], i));
+        if ("proto_value" in this) ret.push(f(this.proto_value, "__proto__"));
         return ret;
     },
     clone: function() {
         var ret = new Dictionary();
-        for (var i in this._values)
-            ret._values[i] = this._values[i];
-        ret._size = this._size;
+        this.each(function(value, i) {
+            ret.set(i, value);
+        });
         return ret;
     },
-    toObject: function() { return this._values }
+    toObject: function() {
+        var obj = {};
+        this.each(function(value, i) {
+            obj["$" + i] = value;
+        });
+        return obj;
+    },
 };
 Dictionary.fromObject = function(obj) {
     var dict = new Dictionary();
-    dict._size = merge(dict._values, obj);
+    for (var i in obj)
+        if (HOP(obj, i)) dict.set(i.slice(1), obj[i]);
     return dict;
 };
 
diff --git a/test/compress/objects.js b/test/compress/objects.js
index 3c7f2d18..beec4bfd 100644
--- a/test/compress/objects.js
+++ b/test/compress/objects.js
@@ -198,9 +198,9 @@ numeric_literal: {
     expect_exact: [
         'var obj = {',
         '    0: 0,',
-        '    "-0": 1,',
-        '    42: 3,',
         '    37: 4,',
+        '    42: 3,',
+        '    "-0": 1,',
         '    o: 5,',
         '    1e42: 8,',
         '    b: 7',