[runtime] Add PositiveNumberToUint32 helper to avoid double to uint roundtrip

test/mjsunit/es6/string-startswith.js

Index: test/mjsunit/es6/string-startswith.js
diff --git a/test/mjsunit/es6/string-startswith.js b/test/mjsunit/es6/string-startswith.js
index 449d07a56d18cbe67428bdef3fdf818c82a5e256..9f4f5838c008c7ffe762baf51730e9e6ef1c6d46 100644
--- a/test/mjsunit/es6/string-startswith.js
+++ b/test/mjsunit/es6/string-startswith.js
@@ -24,6 +24,7 @@
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+assertFalse("abc".startsWith("a", Infinity));
 
 assertEquals(1, String.prototype.startsWith.length);