Fix Tech's sourceHandler deferring to work with IE

src/js/tech/tech.js

@@ -572,32 +572,25 @@ Tech.withSourceHandlers = function(_Tech){
    * When using a source handler, prefer its implementation of
    * any function normally provided by the tech.
    */
-  let techProto = _Tech.prototype;
   let deferrable = [
       'seekable',
       'duration'
     ];
 
-  Object.keys(techProto)
-    .filter(function onlyFunctions(key) {
-        return typeof this[key] === 'function';
-      }, techProto)
-    .filter(function onlyDeferrableKeys(key) {
-        return Boolean(deferrable.indexOf(key) + 1);
-      }, techProto)
-    .map(function getFunctions(key) {
-        return this[key];
-      }, techProto)
-    .forEach(function wrapFunctions(originalFn) {
-        let fnName = originalFn.name;
-
-        this[fnName] = function() {
-          if (this.sourceHandler_ && this.sourceHandler_[fnName]) {
-            return this.sourceHandler_[fnName].apply(this.sourceHandler_, arguments);
-          }
-          return originalFn.apply(this, arguments);
-        };
-      }, techProto);
+  deferrable.forEach(function (fnName) {
+    let originalFn = this[fnName];
+
+    if (typeof originalFn !== 'function') {
+      return;
+    }
+
+    this[fnName] = function() {
+      if (this.sourceHandler_ && this.sourceHandler_[fnName]) {
+        return this.sourceHandler_[fnName].apply(this.sourceHandler_, arguments);
+      }
+      return originalFn.apply(this, arguments);
+    };
+  }, _Tech.prototype);
 
    /*
     * Create a function for setting the source using a source object