compress undefined property names (#2811)

- enforce property names as string
- handle `void 0` as `undefined` in `hoist_props` & `reduce_vars`

lib/parse.js

@@ -1365,7 +1365,7 @@ function parse($TEXT, options) {
             if (type == "name" && !is("punc", ":")) {
                 var key = new AST_SymbolAccessor({
                     start: S.token,
-                    name: as_property_name(),
+                    name: "" + as_property_name(),
                     end: prev()
                 });
                 if (name == "get") {
@@ -1391,7 +1391,7 @@ function parse($TEXT, options) {
             a.push(new AST_ObjectKeyVal({
                 start : start,
                 quote : start.quote,
-                key   : name,
+                key   : "" + name,
                 value : expression(false),
                 end   : prev()
             }));