Add Block and Line classes

lib/models/documents/nodes/block.dart

@@ -0,0 +1,6 @@
+import 'line.dart';
+import 'node.dart';
+
+class Block extends Container<Line> {
+
+}

lib/models/documents/nodes/leaf.dart

@@ -1,6 +1,5 @@
 import 'node.dart';
 
 /* A leaf node in document tree */
-abstract class Leaf extends Node implements StyledNode {
-
+abstract class Leaf extends Node {
 }

lib/models/documents/nodes/line.dart

@@ -0,0 +1,5 @@
+import 'leaf.dart';
+import 'node.dart';
+
+class Line extends Container<Leaf> {
+}

lib/models/documents/nodes/node.dart

@@ -2,14 +2,16 @@ import 'dart:collection';
 
 import 'package:flutter_quill/models/documents/style.dart';
 
-import 'attribute.dart';
+import '../attribute.dart';
 
 /* node in a document tree */
-abstract class Node extends LinkedListEntry<Node> {}
+class Node extends LinkedListEntry<Node> {
+  Container _parent;
+  Style _style = Style();
 
-abstract class StyledNode implements Node {
   Style get style => _style;
-  Style _style = Style();
+
+  Container get parent => _parent;
 
   void applyAttribute(Attribute attribute) {
     _style = _style.merge(attribute);
@@ -25,17 +27,34 @@ abstract class StyledNode implements Node {
   void clearStyle() {
     _style = Style();
   }
-}
-
-abstract class Container<T extends Node> extends Node {
-  Container _parent;
-
-  Container get parent => _parent;
 
   bool get isFirst => list.first == this;
 
   bool get isLast => list.last == this;
+
+  @override
+  void insertBefore(Node entry) {
+    assert(entry._parent == null && _parent != null);
+    entry._parent = _parent;
+    super.insertBefore(entry);
+  }
+
+  @override
+  void insertAfter(Node entry) {
+    assert(entry._parent == null && _parent != null);
+    entry._parent = _parent;
+    super.insertAfter(entry);
+  }
+
+  @override
+  void unlink() {
+    assert(_parent != null);
+    _parent = null;
+    super.unlink();
+  }
 }
 
+abstract class Container<T extends Node> extends Node {}
+
 /* Root node of document tree */
 class Root extends Container<Container<Node>> {}