Changing 'BigInt representing' to 'BigInt value that represents' to k…

…eep consistency and adding missing 'littleEndian' check in 'DataView.prototype.getBigInt64'.

spec.html

@@ -1857,7 +1857,7 @@ <h1>BigInt::add ( _x_, _y_ )</h1>
 
         <emu-clause id="sec-numeric-types-bigint-subtract">
           <h1>BigInt::subtract ( _x_, _y_ )</h1>
-          <p>The abstract operation BigInt::subtract with two arguments _x_ and _y_ of type BigInt returns the BigInt representing the difference _x_ minus _y_.</p>
+          <p>The abstract operation BigInt::subtract with two arguments _x_ and _y_ of type BigInt returns the BigInt value that represents the difference _x_ minus _y_.</p>
         </emu-clause>
 
         <emu-clause id="sec-numeric-types-bigint-leftShift">
@@ -36964,6 +36964,7 @@ <h1>DataView.prototype.getBigInt64 ( _byteOffset_ [ , _littleEndian_ ] )</h1>
         <p>When the `getBigInt64` method is called with argument _byteOffset_ and optional argument _littleEndian_, the following steps are taken:</p>
         <emu-alg>
           1. Let _v_ be the *this* value.
+          1. If _littleEndian_ is not present, let _littleEndian_ be *undefined*.
           1. Return ? GetViewValue(_v_, _byteOffset_, _littleEndian_, `"BigInt64"`).
         </emu-alg>
       </emu-clause>