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

test/mjsunit/es6/string-startswith.js

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+assertFalse("abc".startsWith("a", Infinity));
 
 assertEquals(1, String.prototype.startsWith.length);
 
 var testString = "Hello World";
 assertTrue(testString.startsWith(""));
 assertTrue(testString.startsWith("Hello"));
 assertFalse(testString.startsWith("hello"));
 assertFalse(testString.startsWith("Hello World!"));
 assertFalse(testString.startsWith(null));
 assertFalse(testString.startsWith(undefined));
@@ skipping 364 matching lines @@
   }, [/./]);
 }, TypeError);
 
 // startsWith does its brand checks with Symbol.match
 var re = /./;
 assertThrows(function() {
   "".startsWith(re);
 }, TypeError);
 re[Symbol.match] = false;
 assertEquals(false, "".startsWith(re));