Use fast path for sliced and external strings in ConvertCase.

test/mjsunit/string-case.js

Index: test/mjsunit/string-case.js
diff --git a/test/mjsunit/string-case.js b/test/mjsunit/string-case.js
index 283e703fc33d31df4a75944c7558c68eef7f2e64..34c2340d33c2ff676549e8a571a149dc9b4b26dc 100644
--- a/test/mjsunit/string-case.js
+++ b/test/mjsunit/string-case.js
@@ -25,7 +25,8 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// Flags: --random-seed=17
+// Flags: --random-seed=17 --allow-natives-syntax
+// Flags: --expose-externalize-string
 
 assertEquals("ΚΟΣΜΟΣ ΚΟΣΜΟΣ".toLowerCase(), "κοσμος κοσμος");
 
@@ -58,6 +59,19 @@ function test(length) {
     strLower += String.fromCharCode(charCodeToLower(c));
     strUpper += String.fromCharCode(charCodeToUpper(c));
   }
+  %FlattenString(strLower);
+  %FlattenString(strUpper);
+  // Sequential string.
+  assertEquals(strLower, str.toLowerCase());
+  assertEquals(strUpper, str.toUpperCase());
+  // Cons string.
+  assertEquals(strLower + strLower, (str + str).toLowerCase());
+  assertEquals(strUpper + strUpper, (str + str).toUpperCase());
+  // Sliced string.
+  assertEquals(strLower.substring(1), str.substring(1).toLowerCase());
+  assertEquals(strUpper.substring(1), str.substring(1).toUpperCase());
+  // External string.
+  externalizeString(str, false);
   assertEquals(strLower, str.toLowerCase());
   assertEquals(strUpper, str.toUpperCase());
 }