Remove the --use_browser_controller flag, now that it is always true.

tools/testing/dart/test_suite.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Classes and methods for enumerating and preparing tests.
  *
  * This library includes:
  *
  * - Creating tests by listing all the Dart files in certain directories,
@@ skipping 1095 matching lines @@
               dartFlags.add('--enable_asserts');
               dartFlags.add("--enable_type_checks");
             }
             dartFlags.addAll(vmOptions);
           }
 
           commandSet.add(CommandBuilder.instance.getContentShellCommand(
               contentShellFilename, fullHtmlPath, contentShellOptions,
               dartFlags, configurationDir));
         } else {
-          assert(configuration['use_browser_controller']);
+          // Use the BrowserController to send the test to a browser process.
           // This command is not actually run, it is used for reproducing
           // the failure.
           args = ['tools/testing/dart/launch_browser.dart',
                   runtime,
                   fullHtmlPath];
           commandSet.add(CommandBuilder.instance.getBrowserTestCommand(
               runtime, fullHtmlPath, TestUtils.dartTestExecutable.toString(),
               args, configurationDir, checkedMode: configuration['checked']));
         }
 
@@ skipping 915 matching lines @@
  * $pass tests are expected to pass
  * $failOk tests are expected to fail that we won't fix
  * $fail tests are expected to fail that we should fix
  * $crash tests are expected to crash that we should fix
  * $timeout tests are allowed to timeout
  * $compileErrorSkip tests are skipped on browsers due to compile-time error
 """;
     print(report);
    }
 }