WeakReference constructor doesn't reject object that is not supported as an Expando key

[sgrekhov]

According to the WeakReference documentation

Not all objects are supported as targets for weak references. The WeakReference constructor will reject any object that is not supported as an Expando key.

But, in fact, it doesn't

    WeakReference wr = WeakReference<String>("Lily was here"); // No error here
    print(wr.target); 

Expando does not work on numbers, strings, booleans, null, dart:ffi pointers, dart:ffi structs, or dart:ffi unions. All of these objects must be rejected by WeakReference constructor

[sgrekhov]

cc @dcharkes

[dcharkes]

Thanks @sgrekhov.

Fix underway: https://dart-review.googlesource.com/c/sdk/+/231481.