fix doc comments in dart:io and collection types

sdk/lib/async/future.dart

Index: sdk/lib/async/future.dart
diff --git a/sdk/lib/async/future.dart b/sdk/lib/async/future.dart
index 595489e890f4adb45498f6a66b00b9551ea629ae..8c7d0ce9d99a972f37bc6af7a50bd8deb926d27b 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -308,7 +308,7 @@ abstract class Future<T> {
    * non-null result of successful futures.
    * This makes it posible to `cleanUp` resources that would otherwise be
    * lost (since the returned future does not provide access to these values).
-   * The [cleanup] function is unused if there is no error.
+   * The [cleanUp] function is unused if there is no error.
    *
    * The call to `cleanUp` should not throw. If it does, the error will be an
    * uncaught asynchronous error.