Add a nullFuture in dart:internal.

This is the future returned by `dart:async` in some cases which used to return `null` pre null-safety.
Code inside the SDK which *needs* to not change its timing of events can recognize the future and skip waiting for it, giving the same "synchronous" behavior as before the null safety change.

Longer term, we need to wean people off depending on the specific interleaving of events.
Maybe we should randomly shuffle our microtask queue.

Change-Id: I8d52e74f0549a3068f5dfaffb28c7894b324c49d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/172641
Commit-Queue: Lasse R.H. Nielsen <[email protected]>
Reviewed-by: Nate Bosch <[email protected]>

Author: lrhn

pkg/front_end/testcases/nnbd/issue42546.dart.outline.expect

@@ -28,19 +28,19 @@ static method main() → dynamic
 
 
 Extra constant evaluation status:
-Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:661:13 -> SymbolConstant(#catchError)
-Evaluated: ListLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:661:13 -> ListConstant(const <Type*>[])
-Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:661:13 -> SymbolConstant(#test)
-Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:698:13 -> SymbolConstant(#whenComplete)
-Evaluated: ListLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:698:13 -> ListConstant(const <Type*>[])
-Evaluated: MapLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:698:13 -> InstanceConstant(const _ImmutableMap<Symbol*, dynamic>{_ImmutableMap._kvPairs: const <dynamic>[]})
-Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:725:13 -> SymbolConstant(#timeout)
-Evaluated: ListLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:725:13 -> ListConstant(const <Type*>[])
-Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:725:13 -> SymbolConstant(#onTimeout)
-Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:622:13 -> SymbolConstant(#then)
-Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:622:13 -> SymbolConstant(#onError)
-Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:709:13 -> SymbolConstant(#asStream)
-Evaluated: ListLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:709:13 -> ListConstant(const <Type*>[])
-Evaluated: ListLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:709:13 -> ListConstant(const <dynamic>[])
-Evaluated: MapLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:709:13 -> InstanceConstant(const _ImmutableMap<Symbol*, dynamic>{_ImmutableMap._kvPairs: const <dynamic>[]})
+Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:664:13 -> SymbolConstant(#catchError)
+Evaluated: ListLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:664:13 -> ListConstant(const <Type*>[])
+Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:664:13 -> SymbolConstant(#test)
+Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:701:13 -> SymbolConstant(#whenComplete)
+Evaluated: ListLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:701:13 -> ListConstant(const <Type*>[])
+Evaluated: MapLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:701:13 -> InstanceConstant(const _ImmutableMap<Symbol*, dynamic>{_ImmutableMap._kvPairs: const <dynamic>[]})
+Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:728:13 -> SymbolConstant(#timeout)
+Evaluated: ListLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:728:13 -> ListConstant(const <Type*>[])
+Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:728:13 -> SymbolConstant(#onTimeout)
+Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:625:13 -> SymbolConstant(#then)
+Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:625:13 -> SymbolConstant(#onError)
+Evaluated: SymbolLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:712:13 -> SymbolConstant(#asStream)
+Evaluated: ListLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:712:13 -> ListConstant(const <Type*>[])
+Evaluated: ListLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:712:13 -> ListConstant(const <dynamic>[])
+Evaluated: MapLiteral @ org-dartlang-sdk:///sdk/lib/async/future.dart:712:13 -> InstanceConstant(const _ImmutableMap<Symbol*, dynamic>{_ImmutableMap._kvPairs: const <dynamic>[]})
 Extra constant evaluation: evaluated: 61, effectively constant: 15

sdk/lib/async/async.dart

@@ -101,6 +101,7 @@ import "dart:_internal"
         checkNotNullable,
         EmptyIterator,
         IterableElementError,
+        nullFuture,
         printToZone,
         printToConsole,
         Since,

sdk/lib/async/future.dart

@@ -147,8 +147,11 @@ abstract class FutureOr<T> {
  */
 abstract class Future<T> {
   /// A `Future<Null>` completed with `null`.
-  static final _Future<Null> _nullFuture =
-      new _Future<Null>.zoneValue(null, _rootZone);
+  ///
+  /// Currently shared with `dart:internal`.
+  /// If that future can be removed, then change this back to
+  /// `_Future<Null>.zoneValue(null, _rootZone);`
+  static final _Future<Null> _nullFuture = nullFuture as _Future<Null>;
 
   /// A `Future<bool>` completed with `false`.
   static final _Future<bool> _falseFuture =

sdk/lib/internal/internal.dart

@@ -119,6 +119,28 @@ int parseHexByte(String source, int index) {
   return digit1 * 16 + digit2 - (digit2 & 256);
 }
 
+/// A resusable `null`-valued future used by `dart:async`.
+///
+/// **DO NOT USE.**
+///
+/// This future is used in situations where a future is expected,
+/// but no asynchronous computation actually happens,
+/// like cancelling a stream from a controller with no `onCancel` callback.
+/// *Some code depends on recognizing this future in order to react
+/// synchronously.*
+/// It does so to avoid changing event interleaving during the null safety
+/// migration where, for example, the [StreamSubscription.cancel] method
+/// stopped being able to return `null`.
+/// The code that would be broken by such a timing change is fragile,
+/// but we are not able to simply change it.
+/// For better or worse, code depends on the precise timing that our libraries
+/// have so far exhibited.
+///
+/// This future will be removed again if we can ever do so.
+/// Do not use it for anything other than preserving timing
+/// during the null safety migration.
+final Future<Null> nullFuture = Zone.root.run(() => Future<Null>.value(null));
+
 /// A default hash function used by the platform in various places.
 ///
 /// This is currently the [Jenkins hash function][1] but using masking to keep