Fix engine in preparation for implementing dart-lang/language#1274 (flutter#23131)

When dart-lang/language#1274 (Infer
non-nullability from local boolean variables) is implemented, flow
analysis will detect that code like this no longer needs to perform a
null check:

    final bool hasIdentityTransform =
        transform == null || isIdentityFloat32ListTransform(transform);
    ...
    if (!hasIdentityTransform) {
      ... transform! ... // Null check unnecessary
    }

To avoid a build failure due to the unnecessary null check, we need to
temporarily write it in a way that we can ignore it.  Once the feature
is complete and rolled into flutter, I'll remove the null check
entirely.

Author: stereotype441

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(

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;
     }