Fix typo (dart-archive/async#262)

pkgs/async/lib/src/cancelable_operation.dart

@@ -69,7 +69,7 @@ class CancelableOperation<T> {
   ///
   /// Once any of [operations] completes, its result is forwarded to the
   /// new [CancelableOperation] and the rest are cancelled. If the
-  /// bew operation is cancelled, all the [operations] are cancelled as
+  /// new operation is cancelled, all the [operations] are cancelled as
   /// well.
   static CancelableOperation<T> race<T>(
       Iterable<CancelableOperation<T>> operations) {

pkgs/async/lib/src/result/result.dart

@@ -17,8 +17,8 @@ import 'value.dart';
 /// Capturing a result (either a returned value or a thrown error) means
 /// converting it into a [Result] - either a [ValueResult] or an [ErrorResult].
 ///
-/// This value can release itself by writing itself either to a [EventSink] or a
-/// [Completer], or by becoming a [Future].
+/// This value can release itself by writing itself either to an [EventSink] or
+/// a [Completer], or by becoming a [Future].
 ///
 /// A [Future] represents a potential result, one that might not have been
 /// computed yet, and a [Result] is always a completed and available result.

pkgs/async/lib/src/subscription_stream.dart

@@ -14,7 +14,7 @@ import 'delegate/stream_subscription.dart';
 /// then it is resumed and the events are passed on to the
 /// stream's new subscription.
 ///
-/// This class assumes that is has control over the original subscription.
+/// This class assumes that it has control over the original subscription.
 /// If other code is accessing the subscription, results may be unpredictable.
 class SubscriptionStream<T> extends Stream<T> {
   /// The subscription providing the events for this stream.