[Bug Fix] Concrete class constructor is no longer invoked on extend

- The ctor must be instantiated for use in the prototype chain
- This was working in the past, but apparently no test existed for it and
  refactoring broke it

lib/class_builder.js

@@ -327,6 +327,14 @@ function createConcreteCtor( cname, members )
         initInstance( instance_id, this );
         this.__initProps();
 
+        // If we're extending, we don't actually want to invoke any class
+        // construction logic. The above is sufficient to use this class in a
+        // prototype, so stop here.
+        if ( extending )
+        {
+            return;
+        }
+
         // call the constructor, if one was provided
         if ( this.__construct instanceof Function )
         {

test/test-class-extend.js

@@ -348,3 +348,28 @@ for ( var i = 0; i < class_count; i++ )
     }, Error, "Cannot redeclare method in same class definition" );
 } )();
 
+
+/**
+ * To understand this test, one must understand how "inheritance" works
+ * with prototypes. We must create a new instance of the ctor (class) and add
+ * that instance to the prototype chain (if we added an un-instantiated
+ * constructor, then the members in the prototype would be accessible only
+ * though ctor.prototype). Therefore, when we instantiate this class for use in
+ * the prototype, we must ensure the constructor is not invoked, since our
+ * intent is not to create a new instance of the class.
+ */
+( function testConstructorShouldNotBeCalledWhenExtendingClass()
+{
+    var called = false,
+        Foo    = Class( {
+            'public __construct': function()
+            {
+                called = true;
+            }
+        } ).extend( {} );
+
+    assert.equal( called, false,
+        "Constructor should not be called when extending a class"
+    );
+} )();
+