fix: Allow muxjs to be loaded after Shaka

In PR #2683, a dependency-injection system was added to Shaka Player.
However, it would capture default deps from a global scope at
load-time.  This introduced an accidental ordering requirement where
muxjs would have to be loaded first.  This was not required in v3.0.x,
so it should not have become a requirement in v3.1.

To fix this issue, dependencies are now stored as lazy callbacks.  So
long as muxjs is loaded before Shaka Player tries to play TS content,
it no longer matters when they each were loaded into the page.  This
restores the v3.0.x behavior without breaking the dependency-injection
system or its API.

Closes #3407

Change-Id: Ia6e0a68bea88f0cdb614e8751edc70147bc67f28

Author: joeyparrish

lib/dependencies/all.js

@@ -14,22 +14,23 @@ shaka.dependencies = class {
    * Registers a new dependency.
    *
    * @param {shaka.dependencies.Allowed} key which is used for retrieving a
-   * dependency
+   *   dependency
    * @param {?} dep a dependency
    * @export
    */
   static add(key, dep) {
     if (!shaka.dependencies.Allowed[key]) {
       throw new Error(`${key} is not supported`);
     }
-    shaka.dependencies.dependencies_.set(key, dep);
+    shaka.dependencies.dependencies_.set(key, () => dep);
   }
 
   /**
-   * Check if we have a dependency for the key
-   * @export
+   * Check if we have a dependency for the key.
+   *
    * @param {shaka.dependencies.Allowed} key key
    * @return {boolean}
+   * @export
    */
   static has(key) {
     return shaka.dependencies.dependencies_.has(key);
@@ -38,16 +39,10 @@ shaka.dependencies = class {
   /** @return {?muxjs} */
   static muxjs() {
     return /** @type {?muxjs} */ (shaka.dependencies.dependencies_.get(
-        shaka.dependencies.Allowed.muxjs));
+        shaka.dependencies.Allowed.muxjs)());
   }
 };
 
-/**
- * Contains shared dependencies that could  be used by other components.
- * @private {!Map.<string, Object>}
- */
-shaka.dependencies.dependencies_ = new Map();
-
 /**
  * @export
  * @enum {string}
@@ -56,7 +51,12 @@ shaka.dependencies.Allowed = {
   muxjs: 'muxjs',
 };
 
-// Add global muxjs object for backward compatibility
-shaka.dependencies.dependencies_.set(
-    shaka.dependencies.Allowed.muxjs,
-    window.muxjs);
+/**
+ * Contains accessor functions to shared dependencies that could be used by
+ * other components.  The default accessors can be overridden.
+ *
+ * @private {!Map.<shaka.dependencies.Allowed, function():?>}
+ */
+shaka.dependencies.dependencies_ = new Map([
+  [shaka.dependencies.Allowed.muxjs, () => window.muxjs],
+]);