[dart:async] Temporarily work around compilation errors

The front end is failing to consistently find the correct LUB for function types
with generic type arguments (especially with nullability).

We should revert this change after it is fixed.
dart-lang/sdk#39534

Change-Id: Ie7f6856ccc269c6b287a1f96f264bdd53a2d1782
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/126404
Reviewed-by: Leaf Petersen <leafp@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

Author: nshahan

sdk_nnbd/lib/async/zone.dart

@@ -110,14 +110,16 @@ abstract class ZoneSpecification {
       ForkHandler? fork}) {
     return new ZoneSpecification(
         handleUncaughtError: handleUncaughtError ?? other.handleUncaughtError,
-        run: run ?? other.run,
-        runUnary: runUnary ?? other.runUnary,
-        runBinary: runBinary ?? other.runBinary,
-        registerCallback: registerCallback ?? other.registerCallback,
+        // TODO(#39534) Cleanup casts to dynamic when CFE can find the LUB.
+        run: (run ?? other.run) as dynamic,
+        runUnary: (runUnary ?? other.runUnary) as dynamic,
+        runBinary: (runBinary ?? other.runBinary) as dynamic,
+        registerCallback:
+            (registerCallback ?? other.registerCallback) as dynamic,
         registerUnaryCallback:
-            registerUnaryCallback ?? other.registerUnaryCallback,
+            (registerUnaryCallback ?? other.registerUnaryCallback) as dynamic,
         registerBinaryCallback:
-            registerBinaryCallback ?? other.registerBinaryCallback,
+            (registerBinaryCallback ?? other.registerBinaryCallback) as dynamic,
         errorCallback: errorCallback ?? other.errorCallback,
         scheduleMicrotask: scheduleMicrotask ?? other.scheduleMicrotask,
         createTimer: createTimer ?? other.createTimer,