improve DDC's type checks

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart

 // Copyright (c) 2015, 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.
 
 /// This library adapts ES6 generators to implement Dart's async/await.
 /// It's designed to interact with Dart's Future/Stream and follow Dart
 /// async/await semantics.
 /// See https://github.com/dart-lang/sdk/issues/27315 for ideas on
 /// reconciling Dart's Future and ES6 Promise.
 /// Inspired by `co`: https://github.com/tj/co/blob/master/index.js, which is a
@@ skipping 167 matching lines @@
       if (this.isSuspendedAtYield || this.isAdding) return;
 
       // Handle `await`: if we get a value passed to `yield` it means we are
       // waiting on this Future. Make sure to prevent scheduling, and pass the
       // value back as the result of the `yield`.
       //
       // TODO(jmesserly): is the timing here correct? The assumption here is
       // that we should schedule `await` in `async*` the same as in `async`.
       this.isWaiting = true;
       let future = iter.value;
-      if (!$instanceOf(future, ${getGenericClass(Future)})) {
+      // TODO(jmesserly): `async` uses a different check that looks for the
+      // (private) implementation type of `Future`, rather than the public type.
+      if (!$Future.is(future)) {
         future = $Future.value(future);
       }
       return future.then($dynamic)((x) => this.runBody(x),
           { onError: (e, s) => this.throwError(e, s) });
     }
 
     // Adds element to stream, returns true if the caller should terminate
     // execution of the generator.
     add(event) {
       // If stream is cancelled, tell caller to exit the async generator.
@@ skipping 38 matching lines @@
       }
       if (!this.controller.hasListener) return;
       this.controller.addError(error, stackTrace);
     }
   }
 ''');
 
 /// Returns a Stream of T implemented by an async* function.
 asyncStar(gen, T, @rest args) => JS('', 'new #(#, #, #).controller.stream',
     _AsyncStarStreamController, gen, T, args);