Add property name mangler

We only touch properties that are present in an object literal, or which are
assigned to.  Example:

    x = { foo: 1 };
    x.bar = 2;
    x["baz"] = 3;
    x[cond ? "qwe" : "asd"] = 4;
    console.log(x.stuff);

The names "foo", "bar", "baz", "qwe" and "asd" will be mangled, and the
resulting mangled names will be used for the same properties throughout the
code.  The "stuff" will not be, since it's just referenced but never
assigned to.

This *will* break most of the code out there, but could work on carefully
written code: do not use eval, do not define methods or properties by
walking an array of names, etc.  Also, a comprehensive list of exclusions
needs to be passed, to avoid mangling properties that are standard in
JavaScript, DOM, used in external libraries etc.

bin/uglifyjs

@@ -67,6 +67,9 @@ You need to pass an argument to this option to specify the name that your module
     .describe("bare-returns", "Allow return outside of functions.  Useful when minifying CommonJS modules.")
     .describe("keep-fnames", "Do not mangle/drop function names.  Useful for code relying on Function.prototype.name.")
     .describe("quotes", "Quote style (0 - auto, 1 - single, 2 - double, 3 - original)")
+    .describe("reserved-file", "File containing reserved names")
+    .describe("mangle-props", "Mangle property names")
+    .describe("prop-cache", "File to hold mangled properties mapping")
 
     .alias("p", "prefix")
     .alias("o", "output")
@@ -91,6 +94,8 @@ You need to pass an argument to this option to specify the name that your module
     .string("comments")
     .string("wrap")
     .string("p")
+    .string("reserved-file")
+    .string("prop-cache")
 
     .boolean("expr")
     .boolean("source-map-include-sources")
@@ -106,6 +111,7 @@ You need to pass an argument to this option to specify the name that your module
     .boolean("noerr")
     .boolean("bare-returns")
     .boolean("keep-fnames")
+    .boolean("mangle-props")
 
     .wrap(80)
 
@@ -153,6 +159,16 @@ if (ARGS.r) {
     if (MANGLE) MANGLE.except = ARGS.r.replace(/^\s+|\s+$/g).split(/\s*,+\s*/);
 }
 
+if (ARGS.reserved_file) (function(){
+    var data = fs.readFileSync(ARGS.reserved_file, "utf8");
+    RESERVED = data = JSON.parse(data);
+    if (data.vars) {
+        MANGLE.except = MANGLE.except
+            ? MANGLE.except.concat(data.vars)
+            : data.vars;
+    }
+})();
+
 if (ARGS.quotes === true) {
     ARGS.quotes = 3;
 }
@@ -242,6 +258,7 @@ var OUTPUT_FILE = ARGS.o;
 var TOPLEVEL = null;
 var P_RELATIVE = ARGS.p && ARGS.p == "relative";
 var SOURCES_CONTENT = {};
+var RESERVED = null;
 
 var SOURCE_MAP = ARGS.source_map ? UglifyJS.SourceMap({
     file: P_RELATIVE ? path.relative(path.dirname(ARGS.source_map), OUTPUT_FILE) : OUTPUT_FILE,
@@ -334,6 +351,33 @@ async.eachLimit(files, 1, function (file, cb) {
         TOPLEVEL = TOPLEVEL.wrap_enclose(arg_parameter_list);
     }
 
+    if (ARGS.mangle_props) (function(){
+        var reserved = RESERVED ? RESERVED.props : null;
+        var cache = null;
+        if (ARGS.prop_cache) {
+            try {
+                cache = fs.readFileSync(ARGS.prop_cache, "utf8");
+                cache = JSON.parse(cache);
+                cache.props = UglifyJS.Dictionary.fromObject(cache.props);
+            } catch(ex) {
+                cache = {
+                    cname: -1,
+                    props: new UglifyJS.Dictionary()
+                };
+            }
+        }
+        TOPLEVEL = UglifyJS.mangle_properties(TOPLEVEL, {
+            reserved: reserved,
+            cache: cache
+        });
+        if (ARGS.prop_cache) {
+            fs.writeFileSync(ARGS.prop_cache, JSON.stringify({
+                cname: cache.cname,
+                props: cache.props.toObject()
+            }, null, 2), "utf8");
+        }
+    })();
+
     var SCOPE_IS_NEEDED = COMPRESS || MANGLE || ARGS.lint;
 
     if (SCOPE_IS_NEEDED) {