[dart2wasm,tfa] Don't infer class of string values in comparisons

In dart2wasm, when a comparison like `x == "hello"` is true, we can't
assume that the class of `x` is the same as the class of `"hello"`:

- If `x` is received from JS, it will be `JSStringImpl`.
- If it's a substring of a `TwoByteString`, it will be `TwoByteString`.
- Otherwise it will be `OneByteString`.

Update `Target` with the new method

```
bool get canInferStringClassAfterEqualityComparison => true;
```

to allow TFA to *not* infer classes of string values after comparisons.

Override the method to return `false` in dart2wasm's `Target`
implementation.

Fixes #59901.
Tested: web/wasm/issue_59901_test
Change-Id: I1a6c8deaf27c54240dd4e821dbd8160914502ad7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/404562
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Ömer Ağacan <omersa@google.com>

tests/web/wasm/issue_59901_test.dart

@@ -0,0 +1,18 @@
+// 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.
+
+import 'dart:js_interop';
+
+import 'package:expect/expect.dart';
+
+void main() {
+  final List<int> codeUnits = [];
+  final jsString = "hello".toJS.toDart;
+  if (jsString == "hello") {
+    for (int i = 0; i < 5; i += 1) {
+      codeUnits.add(jsString.codeUnitAt(i));
+    }
+  }
+  Expect.listEquals(codeUnits, [104, 101, 108, 108, 111]);
+}