fix mangleProperties on identifiers (#1776)

- fix handling of "-Infinity"
- add test case for "-0"

reverts #1481

lib/propmangle.js

@@ -47,6 +47,7 @@ function find_builtins() {
     // NaN will be included due to Number.NaN
     var a = [
         "Infinity",
+        "-Infinity",
         "undefined",
     ];
     [ Object, Array, Function, Number,
@@ -153,7 +154,6 @@ function mangle_properties(ast, options) {
     // only function declarations after this line
 
     function can_mangle(name) {
-        if (!is_identifier(name)) return false;
         if (unmangleable.indexOf(name) >= 0) return false;
         if (reserved.indexOf(name) >= 0) return false;
         if (options.only_cache) {