Don't zone-register async callbacks for every await call in the VM.

runtime/lib/async_patch.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.
 
 import "dart:_internal";
 
+// We need to pass the value as first argument and leave the second and third
+// arguments empty (used for error handling).
+// See vm/ast_transformer.cc for usage.
+Function _asyncThenWrapperHelper(continuation) {
+  // Any function that is used as an asynchronous callback must be registered
+  // in the current Zone. Normally, this is done by the future when a
+  // callback is registered (for example with `.then` or `.catchError`). In our
+  // case we want to reuse the same callback multiple times and therefore avoid
+  // the multiple registrations. For our internal futures (`_Future`) we can
+  // use the shortcut-version of `.then`, and skip the registration. However,
+  // that means that the continuation must be registered by us.
+  //
+  // Furthermore, we know that the root-zone doesn't actually do anything and
+  // we can therefore skip the registration call for it.
+  //
+  // Note, that the continuation accepts up to three arguments. If the current
+  // zone is the root zone, we don't wrap the continuation, and a bad
+  // `Future` implementation could potentially invoke the callback with the
+  // wrong number of arguments.
+  if (Zone.current == Zone.ROOT) return continuation;
+  return Zone.current.registerUnaryCallback((x) => continuation(x, null, null));
+}
+
+// We need to pass the exception and stack trace objects as second and third
+// parameter to the continuation.  See vm/ast_transformer.cc for usage.
+Function _asyncErrorWrapperHelper(continuation) {
+  // See comments of `_asyncThenWrapperHelper`.
+  var errorCallback = (e, s) => continuation(null, e, s);
+  if (Zone.current == Zone.ROOT) return errorCallback;
+  return Zone.current.registerBinaryCallback(errorCallback);
+}
+
+/// Registers the [thenCallback] and [errorCallback] on the given [object].
+///
+/// If [object] is not a future, then it is wrapped into one.
+///
+/// Returns the result of registering with `.then`.
+Future _awaitHelper(
+    var object, Function thenCallback, Function errorCallback) {
+  if (object is! Future) {
+    object = new _Future().._setValue(object);
+  } else if (object is! _Future) {
+    return object.then(thenCallback, onError: errorCallback);
+  }
+  // `object` is a `_Future`.
+  //
+  // Since the callbacks have been registered in the current zone (see
+  // [_asyncThenWrapperHelper] and [_asyncErrorWrapperHelper]), we can avoid
+  // another registration and directly invoke the no-zone-registration `.then`.
+  //
+  // We can only do this for our internal futures (the default implementation of
+  // all futures that are constructed by the `dart:async` library).
+  return object._thenNoZoneRegistration(thenCallback, errorCallback);
+}
+
+// _AsyncStarStreamController is used by the compiler to implement
+// async* generator functions.
+class _AsyncStarStreamController {
+  StreamController controller;
+  Function asyncStarBody;
+  bool isAdding = false;
+  bool onListenReceived = false;
+  bool isScheduled = false;
+  bool isSuspendedAtYield = false;
+  Completer cancellationCompleter = null;
+
+  Stream get stream => controller.stream;
+
+  void runBody() {
+    isScheduled = false;
+    isSuspendedAtYield = false;
+    asyncStarBody();
+  }
+
+  void scheduleGenerator() {
+    if (isScheduled || controller.isPaused || isAdding) {
+      return;
+    }
+    isScheduled = true;
+    scheduleMicrotask(runBody);
+  }
+
+  // Adds element to steam, returns true if the caller should terminate
+  // execution of the generator.
+  //
+  // TODO(hausner): Per spec, the generator should be suspended before
+  // exiting when the stream is closed. We could add a getter like this:
+  // get isCancelled => controller.hasListener;
+  // The generator would translate a 'yield e' statement to
+  // controller.add(e);
+  // suspend;
+  // if (controller.isCancelled) return;
+  bool add(event) {
+    if (!onListenReceived) _fatal("yield before stream is listened to!");
+    if (isSuspendedAtYield) _fatal("unexpected yield");
+    // If stream is cancelled, tell caller to exit the async generator.
+    if (!controller.hasListener) {
+      return true;
+    }
+    controller.add(event);
+    scheduleGenerator();
+    isSuspendedAtYield = true;
+    return false;
+  }
+
+  // Adds the elements of stream into this controller's stream.
+  // The generator will be scheduled again when all of the
+  // elements of the added stream have been consumed.
+  // Returns true if the caller should terminate
+  // execution of the generator.
+  bool addStream(Stream stream) {
+    if (!onListenReceived) _fatal("yield before stream is listened to!");
+    // If stream is cancelled, tell caller to exit the async generator.
+    if (!controller.hasListener) return true;
+    isAdding = true;
+    var whenDoneAdding =
+        controller.addStream(stream as Stream, cancelOnError: false);
+    whenDoneAdding.then((_) {
+      isAdding = false;
+      scheduleGenerator();
+    });
+    return false;
+  }
+
+  void addError(error, stackTrace) {
+    if ((cancellationCompleter != null) && !cancellationCompleter.isCompleted) {
+      // If the stream has been cancelled, complete the cancellation future
+      // with the error.
+      cancellationCompleter.completeError(error, stackTrace);
+      return;
+    }
+    // If stream is cancelled, tell caller to exit the async generator.
+    if (!controller.hasListener) return;
+    controller.addError(error, stackTrace);
+    // No need to schedule the generator body here. This code is only
+    // called from the catch clause of the implicit try-catch-finally
+    // around the generator body. That is, we are on the error path out
+    // of the generator and do not need to run the generator again.
+  }
+
+  close() {
+    if ((cancellationCompleter != null) && !cancellationCompleter.isCompleted) {
+      // If the stream has been cancelled, complete the cancellation future
+      // with the error.
+      cancellationCompleter.complete();
+    }
+    controller.close();
+  }
+
+  _AsyncStarStreamController(this.asyncStarBody) {
+    controller = new StreamController(onListen: this.onListen,
+                                      onResume: this.onResume,
+                                      onCancel: this.onCancel);
+  }
+
+  onListen() {
+    assert(!onListenReceived);
+    onListenReceived = true;
+    scheduleGenerator();
+  }
+
+  onResume() {
+    if (isSuspendedAtYield) {
+      scheduleGenerator();
+    }
+  }
+
+  onCancel() {
+    if (controller.isClosed) {
+      return null;
+    }
+    if (cancellationCompleter == null) {
+      cancellationCompleter = new Completer();
+      scheduleGenerator();
+    }
+    return cancellationCompleter.future;
+  }
+}