Make AST_Class inherit AST_Scope instead of AST_Object

This is one of those days I'd love to use multiple inheritance.

An AST_Class has lots of common with AST_Object, but unfortunately
`instanceof AST_Scope` is used very, very much, and a class has its name
inside its own special pocket scope. This compels me to make AST_Class
inherit Scope instead.

It looks like, although there is much in common with AST_Object,
`instanceof AST_Object` seldom are made, perhaps because it is less
often necessary to traverse an object than a scope.

lib/ast.js

@@ -964,13 +964,27 @@ var AST_ObjectGetter = DEFNODE("ObjectGetter", "static", {
     $documentation: "An object getter property",
 }, AST_ObjectProperty);
 
-var AST_Class = DEFNODE("Class", "name extends", {
+var AST_Class = DEFNODE("Class", "name extends properties", {
     $propdoc: {
         name: "[AST_SymbolClassName?] optional class name.",
         extends: "[AST_Node]? optional parent class",
+        properties: "[AST_ObjectProperty*] array of properties"
     },
     $documentation: "An ES6 class",
-}, AST_Object);
+    _walk: function(visitor) {
+        return visitor._visit(this, function(){
+            if (this.name) {
+                this.name._walk(visitor);
+            }
+            if (this.extends) {
+                this.extends._walk(visitor);
+            }
+            this.properties.forEach(function(prop){
+                prop._walk(visitor);
+            });
+        });
+    },
+}, AST_Scope);
 
 var AST_Symbol = DEFNODE("Symbol", "scope name thedef", {
     $propdoc: {

lib/output.js

@@ -502,12 +502,6 @@ function OutputStream(options) {
         return first_in_statement(output);
     });
 
-    // Not a class, though. It can be alone in a statement although
-    // it extends from AST_Object.
-    PARENS(AST_Class, function() {
-        return false;
-    });
-
     PARENS([ AST_Unary, AST_Undefined ], function(output){
         var p = output.parent();
         return p instanceof AST_PropAccess && p.expression === this;