Expect should still fail for non-asynchronous tests.

chrome/test/data/webui/test_api.js

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview Library providing basic test framework functionality.
  */
 
 /**
  * Namespace for |Test|.
@@ skipping 749 matching lines @@
     // that have enabled content-security-policy.
     var testBody = this[testFunction];    // global object -- not a method.
     var testName = testFunction;
     if (typeof testBody === "undefined") {
       testBody = eval(testFunction);
       testName = testBody.toString();
     }
     if (testBody != RUN_TEST_F) {
       console.log('Running test ' + testName);
     }
-    var result = runTestFunction(testFunction, testBody, testArguments, true);
+
+    // Async allow expect errors, but not assert errors.
+    var result = runTestFunction(testFunction, testBody, testArguments,
+                                 isAsync);
     if (!isAsync || !result[0])
       testDone(result);
     return true;
   }
 
   /**
    * This is the guts of WebUIBrowserTest. It runs the test surrounded by an
    * expect to catch Errors. If |errors| is non-empty, it reports a failure and
    * a message by joining |errors|. Consumers can use this to use assert/expect
    * functions asynchronously, but are then responsible for reporting errors to
@@ skipping 518 matching lines @@
   window.SaveMockArguments = SaveMockArguments;
   window.DUMMY_URL = DUMMY_URL;
   window.TEST = TEST;
   window.TEST_F = TEST_F;
   window.GEN = GEN;
   window.WhenTestDone = WhenTestDone;
 
   // Import the Mock4JS helpers.
   Mock4JS.addMockSupport(window);
 })(('window' in this) ? window : this);