Add stack-based messages to all assertions for IE

IE11 does not show stack traces on failed assertions in the console.
So if there is no message associated with a failed assertion, create
one based on the stack trace.  This should help debug errors on IE.

Note that in Shaka 2, we already require messages for all assertions,
so this is just a stop-gap measure to fix lingering bugs in Shaka 1.

Issue #251

Change-Id: Ib42c693278485fa2d3bad7e34f3c73d15d23be27

lib/debug/asserts.js

@@ -30,7 +30,7 @@ goog.provide('shaka.asserts');
 goog.define('shaka.asserts.ENABLE_ASSERTS', goog.DEBUG);
 
 
-/** @type {function()|function(*, string=)} */
+/** @type {function(*, string=)} */
 shaka.asserts.assert = function() {};
 
 
@@ -48,11 +48,6 @@ shaka.asserts.patchAssert_ = function() {
 
   if (!assert) {
     console.assert = function() {};
-  } else if (!assert.bind) {
-    // IE 9 does not provide a .bind for the built-in console functions.
-    console.assert = function() {
-      assert.apply(console, arguments);
-    };
   }
 };
 
@@ -61,8 +56,21 @@ shaka.asserts.patchAssert_ = function() {
 if (shaka.asserts.ENABLE_ASSERTS) {
   shaka.asserts.patchAssert_();
 
-  shaka.asserts.assert =
-      console.assert.bind(console);
+  shaka.asserts.assert = function(test, opt_message) {
+    var message = opt_message;
+    if (!message) {
+      // IE11 does not display any stack information on assertions.  So if
+      // there is no message, create one from the stack.
+      try {
+        throw new Error();
+      } catch (error) {
+        // Line 0 is "Error", line 1 is this location, line 2 is the immediate
+        // caller of shaka.asserts.assert().
+        message = error.stack.split('\n')[2];
+      }
+    }
+    console.assert(test, message);
+  };
 
   shaka.asserts.notImplemented =
       console.assert.bind(console, 0 == 1, 'Not implemented.');