Add an assert and remove a TODO (#7368)

The assert helped me debug an issue recently.

The TODO is obsolete.

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

@@ -517,7 +517,6 @@ abstract class SchedulerBinding extends BindingBase {
   ///
   /// This is only valid while [handleBeginFrame] is running, i.e. while a frame
   /// is being produced.
-  // TODO(ianh): Replace this when fixing https://github.com/flutter/flutter/issues/5469
   Duration get currentFrameTimeStamp {
     assert(_currentFrameTimeStamp != null);
     return _currentFrameTimeStamp;

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

@@ -2790,6 +2790,7 @@ class RepaintBoundary extends SingleChildRenderObjectWidget {
   /// The key for the [RepaintBoundary] is derived either from the child's key
   /// (if the child has a non-null key) or from the given `childIndex`.
   factory RepaintBoundary.wrap(Widget child, int childIndex) {
+    assert(child != null);
     Key key = child.key != null ? new ValueKey<Key>(child.key) : new ValueKey<int>(childIndex);
     return new RepaintBoundary(key: key, child: child);
   }