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

runtime/lib/core_patch.dart

 // Copyright (c) 2013, 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:math";
 import "dart:typed_data";
 
 // Equivalent of calling FATAL from C++ code.
 _fatal(msg) native "DartCore_fatal";
 
-// 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.
-void  _asyncCatchHelper(catchFunction, continuation) {
-  catchFunction((e, s) => continuation(null, e, s));
-}
-
 // The members of this class are cloned and added to each class that
 // represents an enum type.
 class _EnumHelper {
   // Declare the list of enum value names private. When this field is
   // cloned into a user-defined enum class, the field will be inaccessible
   // because of the library-specific name suffix. The toString() function
   // below can access it because it uses the same name suffix.
   static const List<String> _enum_names = null;
   String toString() => _enum_names[index];
   int get hashCode => _enum_names[index].hashCode;
 }
 
-
-// _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;
-  }
-}
-
-
 // _SyncIterable and _syncIterator are used by the compiler to
 // implement sync* generator functions. A sync* generator allocates
 // and returns a new _SyncIterable object.
 
 typedef bool SyncGeneratorCallback(Iterator iterator);
 
 class _SyncIterable extends IterableBase {
   // moveNextFn is the closurized body of the generator function.
   final SyncGeneratorCallback moveNextFn;
 
@@ skipping 33 matching lines @@
       if (isYieldEach) {
         // Spec mandates: it is a dynamic error if the class of [the object
         // returned by yield*] does not implement Iterable.
         yieldEachIterator = (current as Iterable).iterator;
         continue;
       }
       return true;
     }
   }
 }