[regexp] implement /ui to mirror the implementation for /i.

src/regexp/regexp-parser.cc

Index: src/regexp/regexp-parser.cc
diff --git a/src/regexp/regexp-parser.cc b/src/regexp/regexp-parser.cc
index 1e6a0dacca638881cabf0470bf2798c0c53bc0cb..250d03462906b046c1f66c63500bd4e2714cf43a 100644
--- a/src/regexp/regexp-parser.cc
+++ b/src/regexp/regexp-parser.cc
@@ -1294,7 +1294,10 @@ bool RegExpBuilder::NeedsDesugaringForUnicode(RegExpCharacterClass* cc) {
 
 bool RegExpBuilder::NeedsDesugaringForIgnoreCase(uc32 c) {
 #ifdef V8_I18N_SUPPORT
-  if (unicode() && ignore_case()) {
+  if (unicode() && ignore_case() && c >= kNonBmpStart) {
+    // BMP characters are handled in the case-insensitive TextEmitPass.
+    // Surrogate code units do not have case equivalents.
+    // Non-BMP characters need to be desugared into two uc16 parts.
     USet* set = uset_open(c, c);
     uset_closeOver(set, USET_CASE_INSENSITIVE);
     uset_removeAllStrings(set);