[regexp] Fix passing all flags to RegExp construction

Do not bail out when passed a flags string with length > 5, use a
meaningful named constant instead.

Found by https://github.com/tc39/test262/pull/997#issuecomment-296963675

BUG=v8:6300

Review-Url: https://codereview.chromium.org/2841633004
Cr-Commit-Position: refs/heads/master@{#44834}

src/objects.cc

@@ -16232,7 +16232,7 @@ JSRegExp::Flags RegExpFlagsFromString(Handle<String> flags, bool* success) {
   JSRegExp::Flags value = JSRegExp::kNone;
   int length = flags->length();
   // A longer flags string cannot be valid.
-  if (length > 5) return JSRegExp::Flags(0);
+  if (length > JSRegExp::FlagCount()) return JSRegExp::Flags(0);
   for (int i = 0; i < length; i++) {
     JSRegExp::Flag flag = JSRegExp::kNone;
     switch (flags->Get(i)) {

src/objects.h

@@ -7414,9 +7414,12 @@ class JSRegExp: public JSObject {
     kSticky = 1 << 3,
     kUnicode = 1 << 4,
     kDotAll = 1 << 5,
+    // Update FlagCount when adding new flags.
   };
   typedef base::Flags<Flag> Flags;
 
+  static int FlagCount() { return FLAG_harmony_regexp_dotall ? 6 : 5; }
+
   DECL_ACCESSORS(data, Object)
   DECL_ACCESSORS(flags, Object)
   DECL_ACCESSORS(source, Object)

test/mjsunit/harmony/regexp-dotall.js

@@ -56,6 +56,12 @@ function toSlowMode(re) {
   assertFalse(re.dotAll);
 }
 
+// Different construction variants with all flags.
+{
+  assertEquals("gimsuy", new RegExp("", "yusmig").flags);
+  assertEquals("gimsuy", new RegExp().compile("", "yusmig").flags);
+}
+
 // Default '.' behavior.
 {
   let re = /^.$/;