fixes #221, initial sync*, async, async* implementation

test/codegen/expect/async_helper.js

+dart_library.library('async_helper', null, /* Imports */[
+  "dart_runtime/dart",
+  'dart/core',
+  'dart/isolate'
+], /* Lazy imports */[
+], function(exports, dart, core, isolate) {
+  'use strict';
+  let dartx = dart.dartx;
+  exports._initialized = false;
+  exports._port = 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 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)) {
+      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;
+      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.asyncStart = asyncStart;
+  exports.asyncEnd = asyncEnd;
+  exports.asyncSuccess = asyncSuccess;
+  exports.asyncTest = asyncTest;
+});