Fix dart2wasm compilation error in Flutter 3.38+ (#1529)

Author: lemz90

packages/core/lib/src/data/build_op.dart

@@ -169,17 +169,17 @@ class BuildOp {
         (onWidgets != null
             ? (tree, placeholder) {
                 final children = onWidgets(tree, [placeholder]);
-                switch (children?.length) {
-                  case null:
-                    return placeholder;
-                  case 0:
-                    return widget0;
-                  case 1:
-                    return children?.first ?? widget0;
-                  default:
-                    throw UnsupportedError(
-                      'onWidgets must return exactly 1 widget, got ${children?.length}',
-                    );
+                final length = children?.length;
+                if (length == null) {
+                  return placeholder;
+                } else if (length == 0) {
+                  return widget0;
+                } else if (length == 1) {
+                  return children!.first;
+                } else {
+                  throw UnsupportedError(
+                    'onWidgets must return exactly 1 widget, got $length',
+                  );
                 }
               }
             : null);

packages/core/test/_.dart

@@ -162,7 +162,13 @@ Future<String> explainWithoutPumping({
   ).explain(built);
 
   str = str.replaceAll(RegExp('String#[^,]+,'), 'String,');
-  return str.replaceAll(RegExp('Uint8List#[0-9a-f]+,'), 'bytes,');
+  str = str.replaceAll(RegExp('Uint8List#[0-9a-f]+,'), 'bytes,');
+
+  // images
+  str = str.replaceAll(', headers: null', '');
+  str = str.replaceAll(', webHtmlElementStrategy: never', '');
+
+  return str;
 }
 
 final _explainMarginRegExp = RegExp(