fix ArraySplice types to more closely match code

There's a bunch of .length checks on the |current| and |previous| args
given to the public calculateSplices() method (which are duplicated when
these params are passed places).

I've updated from {Array} to {!Array} in every case that an unprotected
`.length` check occurs (which would fail if the parameter is `null`).

lib/utils/array-splice.html

@@ -146,19 +146,19 @@
    *   l: The length of the current array
    *   p: The length of the old array
    *
-   * @param {Array} current The current "changed" array for which to
+   * @param {!Array} current The current "changed" array for which to
    * calculate splices.
    * @param {number} currentStart Starting index in the `current` array for
    * which splices are calculated.
    * @param {number} currentEnd Ending index in the `current` array for
    * which splices are calculated.
-   * @param {Array} old The original "unchanged" array to compare `current`
+   * @param {!Array} old The original "unchanged" array to compare `current`
    * against to determine splices.
    * @param {number} oldStart Starting index in the `old` array for
    * which splices are calculated.
    * @param {number} oldEnd Ending index in the `old` array for
    * which splices are calculated.
-   * @return {Array} Returns an array of splice record objects. Each of these
+   * @return {!Array} Returns an array of splice record objects. Each of these
    * contains: `index` the location where the splice occurred; `removed`
    * the array of removed items from this location; `addedCount` the number
    * of items added at this location.
@@ -300,11 +300,11 @@
      *
      * @function
      * @memberof Polymer.ArraySplice
-     * @param {Array} current The "changed" array for which splices will be
+     * @param {!Array} current The "changed" array for which splices will be
      * calculated.
-     * @param {Array} previous The "unchanged" original array to compare
+     * @param {!Array} previous The "unchanged" original array to compare
      * `current` against to determine the splices.
-     * @return {Array} Returns an array of splice record objects. Each of these
+     * @return {!Array} Returns an array of splice record objects. Each of these
      * contains: `index` the location where the splice occurred; `removed`
      * the array of removed items from this location; `addedCount` the number
      * of items added at this location.