[dart2js] Add failing tests for #49599, #55058, #60115.

Bug: #49599
Bug: #55058
Bug: #60115
Change-Id: I5e7022179a0d64b92304aec31a5732bb4b00db16
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/258960
Reviewed-by: Stephen Adams <[email protected]>

Author: fishythefish

tests/web/regress/issue/49599_test.dart

@@ -0,0 +1,21 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// dart2jsOptions=--disable-type-inference
+
+import 'package:expect/expect.dart';
+
+void main() {
+  final foo = Foo();
+  final bar = foo.mapLookup('a');
+  Expect.equals('c', bar);
+}
+
+class Foo {
+  late var lateMap = <String, String>{};
+
+  String mapLookup(String key) {
+    return lateMap.putIfAbsent(key, () => 'c');
+  }
+}

tests/web/regress/issue/55058_test.dart

@@ -0,0 +1,12 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// dart2jsOptions=--disable-type-inference
+
+import 'package:expect/expect.dart';
+
+void main() {
+  var uri = Uri.parse('https://[email protected]/pathsegment');
+  Expect.isFalse(uri.pathSegments.isEmpty);
+}

tests/web/regress/issue/60115_test.dart

@@ -0,0 +1,16 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// dart2jsOptions=--disable-type-inference
+
+import 'package:expect/expect.dart';
+
+void main() {
+  final foo = Foo();
+  Expect.equals('foo', foo.name);
+}
+
+final class Foo {
+  late final String name = 'foo';
+}