Tentative implementation of string slices (hidden under the flag --string-slices).

src/interpreter-irregexp.cc

Index: src/interpreter-irregexp.cc
diff --git a/src/interpreter-irregexp.cc b/src/interpreter-irregexp.cc
index 1c6c52ca89711049d13f8ff3c8285a1166b12ab1..0d3178dffe5205d37e3f432e0ee0f4aa4e2cb532 100644
--- a/src/interpreter-irregexp.cc
+++ b/src/interpreter-irregexp.cc
@@ -635,7 +635,8 @@ bool IrregexpInterpreter::Match(Isolate* isolate,
   AssertNoAllocation a;
   const byte* code_base = code_array->GetDataStartAddress();
   uc16 previous_char = '\n';
-  if (subject->IsAsciiRepresentation()) {
+
+  if (subject->IsAsciiRepresentationUnderneath()) {
     Vector<const char> subject_vector = subject->ToAsciiVector();
     if (start_position != 0) previous_char = subject_vector[start_position - 1];
     return RawMatch(isolate,