[Effen] Make syncChild() support the _new_ child being null.

This is needed for cases where the UINode doesn't have a list of
children (so it doesn't go through the RenderNodeWrapper sync logic
that removes children), but it still has multiple slots, and needs to
support removing nodes from those slots. (For example, removing a
drawer from a ScaffoldContainer.)

Also, expose syncChild (it used to be private) so it can be overridden
in descendants outside fn.dart.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1158563003

sdk/lib/framework/fn2.dart

@@ -93,15 +93,20 @@ abstract class UINode {
   }
 
   // Returns the child which should be retained as the child of this node.
-  UINode _syncChild(UINode node, UINode oldNode, dynamic slot) {
-    assert(node != null);
-    assert(oldNode == null || node._key == oldNode._key);
+  UINode syncChild(UINode node, UINode oldNode, dynamic slot) {
 
     if (node == oldNode) {
-      _traceSync(_SyncOperation.IDENTICAL, node._key);
+      _traceSync(_SyncOperation.IDENTICAL, node == null ? '*null*' : node._key);
       return node; // Nothing to do. Subtrees must be identical.
     }
 
+    if (node == null) {
+      // the child in this slot has gone away
+      removeChild(oldNode);
+      return null;
+    }
+    assert(oldNode == null || node._key == oldNode._key);
+
     // TODO(rafaelw): This eagerly removes the old DOM. It may be that a
     // new component was built that could re-use some of it. Consider
     // syncing the new VDOM against the old one.
@@ -141,7 +146,7 @@ abstract class ContentNode extends UINode {
 
   void _sync(UINode old, dynamic slot) {
     UINode oldContent = old == null ? null : (old as ContentNode).content;
-    content = _syncChild(content, oldContent, slot);
+    content = syncChild(content, oldContent, slot);
     assert(content.root != null);
     root = content.root;
   }
@@ -441,7 +446,7 @@ abstract class OneChildListRenderNodeWrapper extends RenderNodeWrapper {
     UINode oldNode = null;
 
     void sync(int atIndex) {
-      children[atIndex] = _syncChild(currentNode, oldNode, nextSibling);
+      children[atIndex] = syncChild(currentNode, oldNode, nextSibling);
       assert(children[atIndex] != null);
     }
 
@@ -899,7 +904,7 @@ abstract class Component extends UINode {
     _currentlyBuilding = null;
     _currentOrder = lastOrder;
 
-    _built = _syncChild(_built, oldBuilt, slot);
+    _built = syncChild(_built, oldBuilt, slot);
     _dirty = false;
     root = _built.root;
     assert(root != null);