Turn old space cons strings into regular external strings (not short).

test/mjsunit/regress/regress-sliced-external-cons-regexp.js

+// Copyright 2014 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-externalize-string --expose-gc
+
+var re = /(B)/;
+var cons1 = "0123456789" + "ABCDEFGHIJ";
+var cons2 = "0123456789\u1234" + "ABCDEFGHIJ";
+gc();
+gc();  // Promote cons.
+
+try { externalizeString(cons1, false); } catch (e) { }
+try { externalizeString(cons2, true); } catch (e) { }
+
+var slice1 = cons1.slice(1,-1);
+var slice2 = cons2.slice(1,-1);
+for (var i = 0; i < 10; i++) {
+  assertEquals(["B", "B"], re.exec(slice1));
+  assertEquals(["B", "B"], re.exec(slice2));
+}