Fix IndexedDB types

The built-in externs for IndexedDB in Closure Compiler are missing
some type info.  This overrides them to add what's missing.

This also tweaks type information in a few places in lib/offline/ to
match.

This was caught by a compiler upgrade.

Issue #2528

Change-Id: I97096656f53b426067219e2d4e3aa16f32b87188

externs/idb.js

@@ -0,0 +1,24 @@
+/** @license
+ * Copyright 2016 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @fileoverview Externs for IndexedDB to override the ones provided by the
+ * Closure Compiler.
+ *
+ * TODO: contribute fixes to the Closure Compiler externs
+ *
+ * @externs
+ */
+
+/**
+ * @constructor
+ * @extends {IDBRequest}
+ * @suppress {duplicate}
+ * The upstream extern doesn't have the correct type for Result.
+ */
+var IDBOpenDBRequest = function() {};
+
+/** @type {!IDBDatabase} */
+IDBOpenDBRequest.prototype.result;

lib/offline/indexeddb/base_storage_cell.js

@@ -101,9 +101,9 @@ shaka.offline.indexeddb.BaseStorageCell = class {
     /** @type {!Map.<number, shaka.extern.ManifestDB>} */
     const values = new Map();
 
-    await op.forEachEntry(async (/** number */ key, /** ? */ value) => {
+    await op.forEachEntry(async (key, value) => {
       const manifest = await this.convertManifest(value);
-      values.set(key, manifest);
+      values.set(/** @type {number} */(key), manifest);
     });
 
     await op.promise();
@@ -158,12 +158,12 @@ shaka.offline.indexeddb.BaseStorageCell = class {
     const keys = [];
 
     // Write each segment out. When each request completes, the key will
-    // be in |event.target.result| as can be seen in
+    // be in |request.result| as can be seen in
     // https://w3c.github.io/IndexedDB/#key-generator-construct.
     for (const value of values) {
       const request = store.add(value);
       request.onsuccess = (event) => {
-        const key = event.target.result;
+        const key = /** @type {number} */(request.result);
         keys.push(key);
       };
     }

lib/offline/indexeddb/db_operation.js

@@ -62,7 +62,7 @@ shaka.offline.indexeddb.DBOperation = class {
    * Calls the given callback for each entry in the database.  The callback
    * must be synchronous, but this operation happens asynchronously.
    *
-   * @param {function(number, T, !IDBCursorWithValue=):(Promise|undefined)}
+   * @param {function(!IDBKeyType, T, !IDBCursorWithValue=):(Promise|undefined)}
    *   callback
    * @return {!Promise}
    * @template T
@@ -72,16 +72,16 @@ shaka.offline.indexeddb.DBOperation = class {
       const req = this.store_.openCursor();
       req.onerror = reject;
       req.onsuccess = async (event) => {
-        // When we reach the end of the data that the cursor is iterating
-        // over, |event.target.result| will be null to signal the end of the
-        // iteration.
+        // When we reach the end of the data that the cursor is iterating over,
+        // |req.result| will be null to signal the end of the iteration.
         // https://developer.mozilla.org/en-US/docs/Web/API/IDBCursor/continue
-        const cursor = event.target.result;
-        if (!cursor) {
+        if (req.result == null) {
           resolve();
           return;
         }
 
+        /** @type {!IDBCursorWithValue} */
+        const cursor = /** @type {!IDBCursorWithValue} */(req.result);
         await callback(cursor.key, cursor.value, cursor);
         cursor.continue();
       };

lib/offline/indexeddb/storage_mechanism.js

@@ -54,7 +54,7 @@ shaka.offline.indexeddb.StorageMechanism = class {
     const p = new shaka.util.PublicPromise();
     const open = window.indexedDB.open(name, version);
     open.onsuccess = (event) => {
-      const db = event.target.result;
+      const db = open.result;
       this.db_ = db;
       this.v1_ = shaka.offline.indexeddb.StorageMechanism.createV1_(db);
       this.v2_ = shaka.offline.indexeddb.StorageMechanism.createV2_(db);
@@ -69,7 +69,7 @@ shaka.offline.indexeddb.StorageMechanism = class {
     };
     open.onupgradeneeded = (event) => {
       // Add object stores for the latest version only.
-      this.createStores_(event.target.result);
+      this.createStores_(open.result);
     };
     open.onerror = (event) => {
       p.reject(new shaka.util.Error(