compress undefined property names (#2811)

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

lib/ast.js

@@ -689,8 +689,8 @@ var AST_Object = DEFNODE("Object", "properties", {
 var AST_ObjectProperty = DEFNODE("ObjectProperty", "key value", {
     $documentation: "Base class for literal object properties",
     $propdoc: {
-        key: "[string] the property name converted to a string for ObjectKeyVal.  For setters and getters this is an AST_SymbolAccessor.",
-        value: "[AST_Node] property value.  For setters and getters this is an AST_Accessor."
+        key: "[string|AST_SymbolAccessor] property name. For ObjectKeyVal this is a string. For getters and setters this is an AST_SymbolAccessor.",
+        value: "[AST_Node] property value.  For getters and setters this is an AST_Accessor."
     },
     _walk: function(visitor) {
         return visitor._visit(this, function(){