exclude mangling of special property names (#1779)

- `null`
- `true`
- `false`
- numeric literals

lib/propmangle.js

@@ -46,6 +46,9 @@
 function find_builtins() {
     // NaN will be included due to Number.NaN
     var a = [
+        "null",
+        "true",
+        "false",
         "Infinity",
         "-Infinity",
         "undefined",
@@ -159,7 +162,7 @@ function mangle_properties(ast, options) {
         if (options.only_cache) {
             return cache.props.has(name);
         }
-        if (/^[0-9.]+$/.test(name)) return false;
+        if (/^-?[0-9]+(\.[0-9]+)?(e[+-][0-9]+)?$/.test(name)) return false;
         return true;
     }