fixes #292, report async_helper test failures

test/codegen/expect/async_helper.js

 dart_library.library('async_helper', null, /* Imports */[
   "dart_runtime/dart",
-  'dart/core',
-  'dart/isolate'
+  'dart/core'
 ], /* Lazy imports */[
-], function(exports, dart, core, isolate) {
+], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   exports._initialized = false;
-  exports._port = null;
+  exports._onAsyncEnd = null;
   exports._asyncLevel = 0;
   function _buildException(msg) {
     return core.Exception.new(`Fatal: ${msg}. This is most likely a bug in your test.`);
   }
   dart.fn(_buildException, core.Exception, [core.String]);
+  function asyncTestInitialize(callback) {
+    exports._asyncLevel = 0;
+    exports._initialized = false;
+    exports._onAsyncEnd = callback;
+  }
+  dart.fn(asyncTestInitialize, dart.void, [dart.functionType(dart.void, [])]);
+  dart.copyProperties(exports, {
+    get asyncTestStarted() {
+      return exports._initialized;
+    }
+  });
   function asyncStart() {
     if (dart.notNull(exports._initialized) && exports._asyncLevel == 0) {
       dart.throw(_buildException('asyncStart() was called even though we are done ' + 'with testing.'));
     }
     if (!dart.notNull(exports._initialized)) {
+      if (exports._onAsyncEnd == null) {
+        dart.throw(_buildException('asyncStart() was called before asyncTestInitialize()'));
+      }
       core.print('unittest-suite-wait-for-done');
       exports._initialized = true;
-      exports._port = isolate.ReceivePort.new();
     }
     exports._asyncLevel = dart.notNull(exports._asyncLevel) + 1;
   }
   dart.fn(asyncStart, dart.void, []);
   function asyncEnd() {
     if (dart.notNull(exports._asyncLevel) <= 0) {
       if (!dart.notNull(exports._initialized)) {
         dart.throw(_buildException('asyncEnd() was called before asyncStart().'));
       } else {
         dart.throw(_buildException('asyncEnd() was called more often than ' + 'asyncStart().'));
       }
     }
     exports._asyncLevel = dart.notNull(exports._asyncLevel) - 1;
     if (exports._asyncLevel == 0) {
-      exports._port.close();
-      exports._port = null;
+      let callback = exports._onAsyncEnd;
+      exports._onAsyncEnd = null;
+      dart.dcall(callback);
       core.print('unittest-suite-success');
     }
   }
   dart.fn(asyncEnd, dart.void, []);
   function asyncSuccess(_) {
     return asyncEnd();
   }
   dart.fn(asyncSuccess, dart.void, [dart.dynamic]);
   function asyncTest(f) {
     asyncStart();
     dart.dsend(f(), 'then', asyncSuccess);
   }
   dart.fn(asyncTest, dart.void, [dart.functionType(dart.dynamic, [])]);
   // Exports:
+  exports.asyncTestInitialize = asyncTestInitialize;
   exports.asyncStart = asyncStart;
   exports.asyncEnd = asyncEnd;
   exports.asyncSuccess = asyncSuccess;
   exports.asyncTest = asyncTest;
 });