Add a test util for dumping and restoring from IDB

This makes it easy to dump and restore databases from IndexedDB. This,
in turn, makes it easy for us to test that all database versions can
be read. Before we close the backward compatibility issue that has
plagued v2.3, we will add tests that use this utility to load DB
snapshots from various older schemas and prove that they can still be
read.

This also adds dumps of all database versions to the assets folder,
including a snapshot of a what our broken upgrade in v2.3.0 did to the
v2 database schema.

Issue #1248

Change-Id: If7e8995f50abbdee67e3fa93e79f07a49582c5e8

lib/util/uint8array_utils.js

@@ -17,6 +17,8 @@
 
 goog.provide('shaka.util.Uint8ArrayUtils');
 
+goog.require('shaka.util.StringUtils');
+
 
 /**
  * @namespace shaka.util.Uint8ArrayUtils
@@ -36,7 +38,7 @@ goog.provide('shaka.util.Uint8ArrayUtils');
  */
 shaka.util.Uint8ArrayUtils.toBase64 = function(arr, opt_padding) {
   // btoa expects a "raw string" where each character is interpreted as a byte.
-  let bytes = String.fromCharCode.apply(null, arr);
+  let bytes = shaka.util.StringUtils.fromCharCode(arr);
   let padding = (opt_padding == undefined) ? true : opt_padding;
   let base64 = window.btoa(bytes).replace(/\+/g, '-').replace(/\//g, '_');
   return padding ? base64 : base64.replace(/=*$/, '');