refactor: Add utility to keep unused parameters

lib/util/config_utils.js

@@ -127,4 +127,29 @@ shaka.util.ConfigUtils = class {
     last[fieldName.substring(nameStart).replace(/\\\./g, '.')] = value;
     return configObject;
   }
+
+  /**
+   * Reference the input parameters so the compiler doesn't remove them from
+   * the calling function.  Return whatever value is specified.
+   *
+   * This allows an empty or default implementation of a config callback that
+   * still bears the complete function signature even in compiled mode.
+   *
+   * The caller should look something like this:
+   *
+   *   const callback = (a, b, c, d) => {
+   *     return referenceParametersAndReturn(
+             [a, b, c, d],
+             a);  // Can be anything, doesn't need to be one of the parameters
+   *   };
+   *
+   * @param {!Array.<?>} parameters
+   * @param {T} returnValue
+   * @return {T}
+   * @template T
+   * @noinline
+   */
+  static referenceParametersAndReturn(parameters, returnValue) {
+    return parameters && returnValue;
+  }
 };

lib/util/player_configuration.js

@@ -16,14 +16,6 @@ goog.require('shaka.util.ManifestParserUtils');
 goog.require('shaka.util.Platform');
 
 
-// TODO(vaage): Many times in our configs, we need to create an empty
-//  implementation of a method, but to avoid closure from removing unused
-//  parameters (and breaking our merge config code) we need to use each
-//  parameter. Is there a better solution to this problem than what we are
-//  doing now?
-//
-//  NOTE: Chrome App Content Security Policy prohibits usage of new Function()
-
 /**
  * @final
  * @export
@@ -113,11 +105,10 @@ shaka.util.PlayerConfiguration = class {
           'urn:uuid:f239e769-efa3-4850-9c16-a903c6932efb':
             'com.adobe.primetime',
         },
-        // Need some operation in the callback or else closure may remove calls
-        // to the function as it would be a no-op.  The operation can't just be
-        // a log message, because those are stripped in the compiled build.
         manifestPreprocessor: (element) => {
-          return element;
+          return shaka.util.ConfigUtils.referenceParametersAndReturn(
+              [element],
+              element);
         },
       },
       hls: {
@@ -138,7 +129,9 @@ shaka.util.PlayerConfiguration = class {
       // log message, because those are stripped in the compiled build.
       failureCallback: (error) => {
         shaka.log.error('Unhandled streaming error', error);
-        return [error];
+        return shaka.util.ConfigUtils.referenceParametersAndReturn(
+            [error],
+            undefined);
       },
       // When low latency streaming is enabled, rebufferingGoal will default to
       // 0.01 if not specified.
@@ -215,7 +208,9 @@ shaka.util.PlayerConfiguration = class {
       // to the function as it would be a no-op.  The operation can't just be a
       // log message, because those are stripped in the compiled build.
       progressCallback: (content, progress) => {
-        return [content, progress];
+        return shaka.util.ConfigUtils.referenceParametersAndReturn(
+            [content, progress],
+            undefined);
       },
 
       // By default we use persistent licenses as forces errors to surface if