Fix a batch of DDC SDK compile errors.

sdk/lib/async/future.dart

 // Copyright (c) 2012, 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.
 
 part of dart.async;
 
 /// A type representing values that are either `Future<T>` or `T`.
 ///
 /// This class declaration is a public stand-in for an internal
 /// future-or-value generic type. References to this class are resolved to the
@@ skipping 117 matching lines @@
  * the `then` call, it would not catch errors from the `bar` call.
  *
  * Futures can have more than one callback-pair registered. Each successor is
  * treated independently and is handled as if it was the only successor.
  *
  * A future may also fail to ever complete. In that case, no callbacks are
  * called.
  */
 abstract class Future<T> {
   // The `_nullFuture` is a completed Future with the value `null`.
-  static final _Future _nullFuture = new Future.value(null);
+  static final _Future<Null> _nullFuture = new Future.value(null);
 
   /**
    * Creates a future containing the result of calling [computation]
    * asynchronously with [Timer.run].
    *
    * If the result of executing [computation] throws, the returned future is
    * completed with the error.
    *
    * If the returned value is itself a [Future], completion of
    * the created future will wait until the returned future completes,
@@ skipping 737 matching lines @@
   AsyncError replacement = Zone.current.errorCallback(error, stackTrace);
   if (replacement != null) {
     error = _nonNullError(replacement.error);
     stackTrace = replacement.stackTrace;
   }
   result._asyncCompleteError(error, stackTrace);
 }
 
 /** Helper function that converts `null` to a [NullThrownError]. */
 Object _nonNullError(Object error) => error ?? new NullThrownError();