Added %FlattenString and use it to speed up a regression test.

test/mjsunit/regress/regress-crbug-160010.js

Index: test/mjsunit/regress/regress-crbug-160010.js
diff --git a/test/mjsunit/regress/regress-crbug-160010.js b/test/mjsunit/regress/regress-crbug-160010.js
index 266e54532597d060b699788fc210d566941fba60..586bddd7c23c38d6218eea69c4c790566c52225b 100644
--- a/test/mjsunit/regress/regress-crbug-160010.js
+++ b/test/mjsunit/regress/regress-crbug-160010.js
@@ -25,9 +25,11 @@
 // (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: --allow-natives-syntax
+
 var str = "a";
 for (var i = 0; i < 28; i++) {
   str += str;
+  %FlattenString(str);  // Evil performance hack
 }
 JSON.stringify(str);
-