diff --git a/lib/web_ui/lib/src/engine/html/surface.dart b/lib/web_ui/lib/src/engine/html/surface.dart
index 3a947bb15dc1b3c059c3f0f8cff29e19b749745b..b01ecbde7fb556613c28c9be5c60016b63eabee6 100644
--- a/lib/web_ui/lib/src/engine/html/surface.dart
+++ b/lib/web_ui/lib/src/engine/html/surface.dart
@@ -1092,7 +1092,17 @@ abstract class PersistedContainerSurface extends PersistedSurface {
       for (int indexInOld = 0; indexInOld < oldChildCount; indexInOld += 1) {
         final PersistedSurface? oldChild = oldChildren[indexInOld];
         final bool childAlreadyClaimed = oldChild == null;
-        if (childAlreadyClaimed || !newChild.canUpdateAsMatch(oldChild!)) {
+        // After https://github.com/dart-lang/language/issues/1274 is
+        // implemented, `oldChild` will be promoted to non-nullable on the RHS
+        // of the `||`, so we won't need to null check it (and it will cause a
+        // build failure to try to do so).  Until then, we need to null check it
+        // in such a way that won't cause a build failure once the feature is
+        // implemented.  We can do that by casting to `dynamic`, and then
+        // relying on the call to `canUpdateAsMatch` implicitly downcasting to
+        // PersistentSurface.
+        // TODO(paulberry): remove this workaround once the feature is
+        // implemented.
+        if (childAlreadyClaimed || !newChild.canUpdateAsMatch(oldChild as dynamic)) {
           continue;
         }
         allMatches.add(_PersistedSurfaceMatch(
diff --git a/lib/web_ui/lib/src/engine/semantics/semantics.dart b/lib/web_ui/lib/src/engine/semantics/semantics.dart
index 1d511e5198cf34b55d9daa5c1bc0219b09126656..62fd421a804480cda84be4c7d2c411e0da988978 100644
--- a/lib/web_ui/lib/src/engine/semantics/semantics.dart
+++ b/lib/web_ui/lib/src/engine/semantics/semantics.dart
@@ -889,7 +889,17 @@ class SemanticsObject {
         effectiveTransformIsIdentity = effectiveTransform.isIdentity();
       }
     } else if (!hasIdentityTransform) {
-      effectiveTransform = Matrix4.fromFloat32List(transform!);
+      // After https://github.com/dart-lang/language/issues/1274 is implemented,
+      // `transform` will be promoted to non-nullable so we won't need to null
+      // check it (and it will cause a build failure to try to do so).  Until
+      // then, we need to null check it in such a way that won't cause a build
+      // failure once the feature is implemented.  We can do that using an
+      // explicit "if" test.
+      // TODO(paulberry): remove this check once the feature is implemented.
+      if (transform == null) { // ignore: unnecessary_null_comparison
+        throw 'impossible';
+      }
+      effectiveTransform = Matrix4.fromFloat32List(transform);
       effectiveTransformIsIdentity = false;
     }