Fixed several typos

packages/flutter/lib/widgets/variable_height_scrollable.dart

@@ -41,8 +41,8 @@ class VariableHeightScrollable extends Scrollable {
     if (layoutState != source.layoutState) {
       // Warning: this is unlikely to be what you intended.
       assert(source.layoutState != null);
-      layoutState == source.layoutState;
       layoutState.removeListener(_handleLayoutChanged);
+      layoutState = source.layoutState;
       layoutState.addListener(_handleLayoutChanged);
     }
     super.syncFields(source);
@@ -55,6 +55,11 @@ class VariableHeightScrollable extends Scrollable {
     scrollBehavior.containerSize = newSize.height;
   }
 
+  void didUnmount() {
+    layoutState.removeListener(_handleLayoutChanged);
+    super.didUnmount();
+  }
+
   void _handleLayoutChanged() {
     if (layoutState.didReachLastChild) {
       scrollBehavior.contentsSize = layoutState.contentsSize;