[gardening] Fix corelib/weak_reference_test

This test incorrectly expects that WeakReference.target should be
cleared for a weakly reachable object. WeakReference specification
doesn't guarantee that:

https://github.com/dart-lang/sdk/blob/a9f684e62499e669294a07c3ce7d2aa7c545760a/sdk/lib/core/weak.dart#L75-L76

This change fixes the test to give up after certain number of
iterations instead of hanging forever.

Issue: #55518
Change-Id: Ief0ebe1452c83058a35fc1ba87e5a83924d43919
TEST=corelib/weak_reference_test
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/363960
Reviewed-by: Siva Annamalai <[email protected]>
Commit-Queue: Siva Annamalai <[email protected]>
Reviewed-by: Ryan Macnak <[email protected]>
Auto-Submit: Alexander Markov <[email protected]>

Author: alexmarkov

tests/corelib/weak_reference_test.dart

@@ -41,10 +41,18 @@ Future testWeakReferenceWeakness() async {
     foo = null;
   }
   asyncStart();
-  while (weakReference.target != null) {
+  // According to the WeakReference specification:
+  //
+  // There are no guarantees that a weak reference will ever be cleared
+  // even if all references to its target are weak references.
+  //
+  // Wait a few iterations and give up if target is not cleared.
+  const int numIterations = 10;
+  int i = 0;
+  for (; weakReference.target != null && i < numIterations; ++i) {
     produceGarbage();
     await Future.delayed(const Duration(milliseconds: 10));
   }
-  Expect.isNull(weakReference.target);
+  Expect.isTrue(i == numIterations || weakReference.target == null);
   asyncEnd();
 }