Enable slices of external strings (in the tentative implementation).

test/mjsunit/string-slices.js

Index: test/mjsunit/string-slices.js
diff --git a/test/mjsunit/string-slices.js b/test/mjsunit/string-slices.js
index 8cc1f81e774fe6b8449555b90a3d5bba868f4db2..40b9a20d1cdb132390d86863e25ad22e0c3d1842 100755
--- a/test/mjsunit/string-slices.js
+++ b/test/mjsunit/string-slices.js
@@ -189,11 +189,16 @@ assertEquals("\u03B4\u03B5\u03B6\u03B7\u03B8\u03B9abcdefghijklmnop",
 assertEquals("\u03B2\u03B3\u03B4\u03B5\u03B4\u03B5\u03B6\u03B7",
     utf.substring(5,1) + utf.substring(3,7));
 
-/*
 // Externalizing strings.
-var a = "123456789qwertyuiopasdfghjklzxcvbnm";
-var b = a.slice(1,-1);
+var a = "123456789" + "qwertyuiopasdfghjklzxcvbnm";
+var b = "23456789qwertyuiopasdfghjklzxcvbn"
 assertEquals(a.slice(1,-1), b);
-externalizeString(a);
+
+assertTrue(isAsciiString(a));
+externalizeString(a, true);
+assertFalse(isAsciiString(a));
+
 assertEquals(a.slice(1,-1), b);
-*/
+assertTrue(/3456789qwe/.test(a));
+assertEquals(5, a.indexOf("678"));
+assertEquals("12345", a.split("6")[0]);