Use ICU case conversion/transliterator for case conversion behind a flag

test/intl/general/case-mapping.js

Index: test/intl/general/case-mapping.js
diff --git a/test/intl/general/case-mapping.js b/test/intl/general/case-mapping.js
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+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Some edge cases that unibrow got wrong
+
+assertEquals("𐐘", "𐑀".toUpperCase());
+assertEquals("𐑀", "𐐘".toLowerCase());
+assertEquals("σ", "Σ".toLowerCase());
+
+
+// Some different paths in the ICU case conversion fastpath
+
+assertEquals("σς", "\u03A3\u03A3".toLowerCase());
+// Expand sharp s in latin1 fastpath
+assertEquals("ASSB", "A\u00DFB".toUpperCase());
+assertEquals("AB", "Ab".toUpperCase());
+// Find first upper case in fastpath
+assertEquals("ab", "aB".toLowerCase());
+assertEquals("AÜ", "aü".toUpperCase());
+assertEquals("AÜ", "AÜ".toUpperCase());
+assertEquals("aü", "aü".toLowerCase());
+assertEquals("aü", "AÜ".toLowerCase());
+assertEquals("aü", "AÜ".toLowerCase());
+// Starts with fastpath, but switches to full Unicode path
+// U+00FF is uppercased to U+0178.
+assertEquals("AŸ", "aÿ".toUpperCase());
+// Buffer size increase
+assertEquals("CSSBẶ", "c\u00DFbặ".toUpperCase());
+assertEquals("FIFLFFIFFL", "\uFB01\uFB02\uFB03\uFB04".toUpperCase());
+// Plane 1; Deseret and Warang Citi Script.
+assertEquals("\u{10400}\u{118A0}", "\u{10428}\u{118C0}".toUpperCase());
+assertEquals("\u{10428}\u{118C0}", "\u{10400}\u{118A0}".toLowerCase());
+// Mathematical Bold {Capital, Small} Letter A do not change.
+assertEquals("\u{1D400}\u{1D41A}", "\u{1D400}\u{1D41A}".toUpperCase());
+assertEquals("\u{1D400}\u{1D41A}", "\u{1D400}\u{1D41A}".toLowerCase());
+// Plane 1; New characters in Unicode 8.0
+assertEquals("\u{10C80}", "\u{10CC0}".toUpperCase());
+assertEquals("\u{10CC0}", "\u{10C80}".toLowerCase());
+assertEquals("\u{10C80}", "\u{10CC0}".toLocaleUpperCase());
+assertEquals("\u{10CC0}", "\u{10C80}".toLocaleLowerCase());
+assertEquals("\u{10C80}", "\u{10CC0}".toLocaleUpperCase(["tr"]));
+assertEquals("\u{10C80}", "\u{10CC0}".toLocaleUpperCase(["tr"]));
+assertEquals("\u{10CC0}", "\u{10C80}".toLocaleLowerCase());