refactor classes, add private, minor fixes

1-js/09-classes/03-class-inheritance/1-class-constructor-error/solution.md

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+That's because the child constructor must call `super()`.
+
+Here's the corrected code:
+
+```js run
+class Animal {
+
+  constructor(name) {
+    this.name = name;
+  }
+
+}
+
+class Rabbit extends Animal {
+  constructor(name) {  
+    *!*
+    super(name);
+    */!*
+    this.created = Date.now();
+  }
+}
+
+*!*
+let rabbit = new Rabbit("White Rabbit"); // ok now
+*/!*
+alert(rabbit.name); // White Rabbit
+```

1-js/09-classes/03-class-inheritance/1-class-constructor-error/task.md

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+importance: 5
+
+---
+
+# Error creating an instance
+
+Here's the code with `Rabbit` extending `Animal`.
+
+Unfortunately, `Rabbit` objects can't be created. What's wrong? Fix it.
+```js run
+class Animal {
+
+  constructor(name) {
+    this.name = name;
+  }
+
+}
+
+class Rabbit extends Animal {
+  constructor(name) {  
+    this.name = name;
+    this.created = Date.now();
+  }
+}
+
+*!*
+let rabbit = new Rabbit("White Rabbit"); // Error: this is not defined
+*/!*
+alert(rabbit.name);
+```