Fix the crash when the drawer is hidden.

The root cause of the crash was threefold:
 - you can't enumerate enums, in dart; you have to enumerate their values.
 - RenderNodeWrapper was trying to call remove(child.root) on its root, but that's
   not something that all RenderNodes have
 - when a RenderNode was deparented, its relayoutSubtreeRoot wasn't removed, so
   the node ended up in an inconsistent state regarding its parent tree

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1151573020

sdk/lib/framework/components2/scaffold.dart

@@ -48,7 +48,7 @@ class RenderScaffold extends RenderBox {
 
   ScaffoldSlots remove(RenderBox child) {
     assert(child != null);
-    for (ScaffoldSlots slot in ScaffoldSlots) {
+    for (ScaffoldSlots slot in ScaffoldSlots.values) {
       if (_slots[slot] == child) {
         this[slot] = null;
         return slot;

sdk/lib/framework/fn2.dart

@@ -317,11 +317,6 @@ abstract class RenderNodeWrapper extends UINode {
     }
   }
 
-  void removeChild(UINode node) {
-    root.remove(node.root);
-    super.removeChild(node);
-  }
-
   void _remove() {
     assert(root != null);
     _nodeMap.remove(root);
@@ -334,20 +329,25 @@ abstract class OneChildRenderNodeWrapper extends RenderNodeWrapper {
 
   OneChildRenderNodeWrapper({ this.child, Object key }) : super(key: key);
 
+  void syncRenderNode(RenderNodeWrapper old) {
+    super.syncRenderNode(old);
+    UINode oldChild = old == null ? null : (old as OneChildRenderNodeWrapper).child;
+    syncChild(child, oldChild, null);
+  }
+
   void insert(RenderNodeWrapper child, dynamic slot) {
     assert(slot == null);
     root.child = child.root;
   }
 
-  void syncRenderNode(RenderNodeWrapper old) {
-    super.syncRenderNode(old);
-    UINode oldChild = old == null ? null : (old as OneChildRenderNodeWrapper).child;
-    syncChild(child, oldChild, null);
+  void removeChild(UINode node) {
+    root.child = null;
+    super.removeChild(node);
   }
 
   void _remove() {
-    assert(child != null);
-    removeChild(child);
+    if (child != null)
+      removeChild(child);
     super._remove();
   }
 }
@@ -436,6 +436,12 @@ abstract class OneChildListRenderNodeWrapper extends RenderNodeWrapper {
     root.add(child.root, before: slot);
   }
 
+  void removeChild(UINode node) {
+    assert(root is ContainerRenderNodeMixin);
+    root.remove(node.root);
+    super.removeChild(node);
+  }
+
   void _remove() {
     assert(children != null);
     for (var child in children) {

sdk/lib/framework/rendering/node.dart

@@ -55,12 +55,18 @@ abstract class RenderNode extends AbstractNode {
     assert(child != null);
     setParentData(child);
     super.adoptChild(child);
+    markNeedsLayout();
   }
   void dropChild(RenderNode child) { // only for use by subclasses
     assert(child != null);
     assert(child.parentData != null);
     child.parentData.detach();
+    if (child._relayoutSubtreeRoot != child) {
+      child._relayoutSubtreeRoot = null;
+      child._needsLayout = true;
+    }
     super.dropChild(child);
+    markNeedsLayout();
   }
 
   static List<RenderNode> _nodesNeedingLayout = new List<RenderNode>();
@@ -231,7 +237,6 @@ abstract class RenderNodeWithChildMixin<ChildType extends RenderNode> {
     _child = value;
     if (_child != null)
       adoptChild(_child);
-    markNeedsLayout();
   }
   void attachChildren() {
     if (_child != null)
@@ -336,7 +341,6 @@ abstract class ContainerRenderNodeMixin<ChildType extends RenderNode, ParentData
         assert(before.parentData.previousSibling == child);
       }
     }
-    markNeedsLayout();
   }
   void remove(ChildType child) {
     assert(child.parentData is ParentDataType);
@@ -359,7 +363,6 @@ abstract class ContainerRenderNodeMixin<ChildType extends RenderNode, ParentData
     child.parentData.previousSibling = null;
     child.parentData.nextSibling = null;
     dropChild(child);
-    markNeedsLayout();
   }
   void redepthChildren() {
     ChildType child = _firstChild;