Rename "detachChild" to "forgetChild" (#6845)

...to avoid confusion with "detachRenderObject" which is basically
unrelated.

packages/flutter/lib/src/widgets/binding.dart

@@ -495,7 +495,7 @@ class RenderObjectToWidgetElement<T extends RenderObject> extends RootRenderObje
   }
 
   @override
-  void detachChild(Element child) {
+  void forgetChild(Element child) {
     assert(child == _child);
     _child = null;
   }

packages/flutter/lib/src/widgets/layout_builder.dart

@@ -70,7 +70,7 @@ class _LayoutBuilderElement extends RenderObjectElement {
   }
 
   @override
-  void detachChild(Element child) {
+  void forgetChild(Element child) {
     assert(child == _child);
     _child = null;
   }

packages/flutter/lib/src/widgets/lazy_block.dart

@@ -514,9 +514,9 @@ class _LazyBlockElement extends RenderObjectElement {
   }
 
   @override
-  void detachChild(Element child) {
+  void forgetChild(Element child) {
     assert(() {
-      // TODO(ianh): implement detachChild for LazyBlock
+      // TODO(ianh): implement forgetChild for LazyBlock
       throw new FlutterError(
         'LazyBlock does not yet support GlobalKey reparenting of its children.\n'
         'As a temporary workaround, wrap the child with the GlobalKey in a '

packages/flutter/lib/src/widgets/overlay.dart

@@ -418,7 +418,7 @@ class _TheatreElement extends RenderObjectElement {
   static final Object _onstageSlot = new Object();
 
   List<Element> _offstage;
-  final Set<Element> _detachedOffstageChildren = new HashSet<Element>();
+  final Set<Element> _forgottenOffstageChildren = new HashSet<Element>();
 
   @override
   void insertChildRenderObject(RenderBox child, dynamic slot) {
@@ -462,7 +462,7 @@ class _TheatreElement extends RenderObjectElement {
     if (_onstage != null)
       visitor(_onstage);
     for (Element child in _offstage) {
-      if (!_detachedOffstageChildren.contains(child))
+      if (!_forgottenOffstageChildren.contains(child))
         visitor(child);
     }
   }
@@ -474,13 +474,13 @@ class _TheatreElement extends RenderObjectElement {
   }
 
   @override
-  bool detachChild(Element child) {
+  bool forgetChild(Element child) {
     if (child == _onstage) {
       _onstage = null;
     } else {
       assert(_offstage.contains(child));
-      assert(!_detachedOffstageChildren.contains(child));
-      _detachedOffstageChildren.add(child);
+      assert(!_forgottenOffstageChildren.contains(child));
+      _forgottenOffstageChildren.add(child);
     }
     return true;
   }
@@ -503,8 +503,8 @@ class _TheatreElement extends RenderObjectElement {
     super.update(newWidget);
     assert(widget == newWidget);
     _onstage = updateChild(_onstage, widget.onstage, _onstageSlot);
-    _offstage = updateChildren(_offstage, widget.offstage, detachedChildren: _detachedOffstageChildren);
-    _detachedOffstageChildren.clear();
+    _offstage = updateChildren(_offstage, widget.offstage, forgottenChildren: _forgottenOffstageChildren);
+    _forgottenOffstageChildren.clear();
   }
 }
 

packages/flutter/lib/src/widgets/table.dart

@@ -274,7 +274,7 @@ class _TableElement extends RenderObjectElement {
     renderObject.setChild(childParentData.x, childParentData.y, null);
   }
 
-  final Set<Element> _detachedChildren = new HashSet<Element>();
+  final Set<Element> _forgottenChildren = new HashSet<Element>();
 
   @override
   void update(Table newWidget) {
@@ -300,17 +300,17 @@ class _TableElement extends RenderObjectElement {
       }
       newChildren.add(new _TableElementRow(
         key: row.key,
-        children: updateChildren(oldChildren, row.children, detachedChildren: _detachedChildren)
+        children: updateChildren(oldChildren, row.children, forgottenChildren: _forgottenChildren)
       ));
     }
     while (oldUnkeyedRows.moveNext())
-      updateChildren(oldUnkeyedRows.current.children, const <Widget>[], detachedChildren: _detachedChildren);
+      updateChildren(oldUnkeyedRows.current.children, const <Widget>[], forgottenChildren: _forgottenChildren);
     for (List<Element> oldChildren in oldKeyedRows.values.where((List<Element> list) => !taken.contains(list)))
-      updateChildren(oldChildren, const <Widget>[], detachedChildren: _detachedChildren);
+      updateChildren(oldChildren, const <Widget>[], forgottenChildren: _forgottenChildren);
     assert(() { _debugWillReattachChildren = false; return true; });
     _children = newChildren;
     _updateRenderObjectChildren();
-    _detachedChildren.clear();
+    _forgottenChildren.clear();
     super.update(newWidget);
     assert(widget == newWidget);
   }
@@ -326,14 +326,14 @@ class _TableElement extends RenderObjectElement {
   @override
   void visitChildren(ElementVisitor visitor) {
     for (Element child in _children.expand((_TableElementRow row) => row.children)) {
-      if (!_detachedChildren.contains(child))
+      if (!_forgottenChildren.contains(child))
         visitor(child);
     }
   }
 
   @override
-  bool detachChild(Element child) {
-    _detachedChildren.add(child);
+  bool forgetChild(Element child) {
+    _forgottenChildren.add(child);
     return true;
   }
 }

packages/flutter/lib/src/widgets/virtual_viewport.dart

@@ -104,9 +104,9 @@ abstract class VirtualViewportElement extends RenderObjectElement {
   }
 
   @override
-  void detachChild(Element child) {
+  void forgetChild(Element child) {
     assert(() {
-      // TODO(ianh): implement detachChild for VirtualViewport
+      // TODO(ianh): implement forgetChild for VirtualViewport
       throw new FlutterError(
         '$runtimeType does not yet support GlobalKey reparenting of its children.\n'
         'As a temporary workaround, wrap the child with the GlobalKey in a '