[benchmarks] Add LongStringCompare benchmark that measure string comparison performance.

BUG=https://github.com/dart-lang/sdk/issues/50190
TEST=ci

Change-Id: I1cb93455283b19cf1a712132920b7d3e1dabcd8a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/269380
Commit-Queue: Alexander Aprelev <aam@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>

benchmarks/StringCompare/dart/LongStringCompare.dart

@@ -0,0 +1,46 @@
+// Copyright (c) 2022, 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.
+//
+// Measure performance of string comparison.
+
+import 'package:benchmark_harness/benchmark_harness.dart';
+
+class LongStringCompare extends BenchmarkBase {
+  late String s, t;
+
+  String generateLongString() {
+    var s = "abcdefgh";
+    for (int i = 0; i < 20; i++) {
+      s = "$s ghijkl $s";
+    }
+    return s;
+  }
+
+  LongStringCompare() : super('LongStringCompare') {
+    s = generateLongString();
+    t = s;
+    // Difference in two strings goes in the middle.
+    s += "." + s;
+    t += "!" + t;
+  }
+
+  @override
+  void warmup() {
+    for (int i = 0; i < 4; i++) {
+      run();
+    }
+  }
+
+  @override
+  void run() {
+    bool b = true;
+    for (int i = 0; i < 5; i++) {
+      b &= (s == t);
+    }
+  }
+}
+
+void main() {
+  LongStringCompare().report();
+}