This restriction comes directly from the language spec. I think the spec would need to be updated before we could update analyzer, so I'm marking this as a language issue.

Seems like a bug/oversight in the specification. I'm sympathetic to fixing it.
There'll likely be a clean-up of the const/potentially const specification for Dart 2, to iron out a few other problematic cases, but I see no reason not to commit to this immediately.

Dup of #30288 ?

The example no longer reports an error:

$ dartanalyzer --version
dartanalyzer version 2.0.0-dev.63.0
$ cat 31140.dart 
class Foo {
  const Foo({ this.child, this.text }) : assert(child == null || text == null);

  final String child;
  final String text;
}
$ dartanalyzer 31140.dart 
Analyzing 31140.dart...
No issues found!

If I try to revert our workaround for this and use assert(child == null || text == null), I get:

[error] In constant expressions, operands of this operator must be of type 'bool' (/usr/local/google/home/ianh/dev/flutter/packages/flutter/lib/src/ma\
terial/tabs.dart:64:15)
[error] In constant expressions, operands of this operator must be of type 'bool', 'num', 'String' or 'null' (/usr/local/google/home/ianh/dev/flutter/\
packages/flutter/lib/src/material/tabs.dart:64:31)

Issue is definitely still here. The problem does not occur in the test above because the constructor is never called with an object which isn't of those types.

FWIW this issue affects Hummingbird. We have to comment out some asserts to make the code run in DDC.

I think analyzer 2.5.0, with the const-update-2018, produces the correct behavior. Using the example above:

class Foo {
  const Foo({ this.child, this.text }) : assert(child == null || text == null);

  final Widget child;
  final String text;
}

class Widget {
  const Widget();
}

void main() {
  const Foo(child: null, text: null);
  const Foo(child: null, text: 'yay');
  const Foo(child: const Widget(), text: null);
  const Foo(child: const Widget(), text: 'yay');
}

The analyzer only reports an error on the last line:

Evaluation of this constant expression throws an exception.

We merged into the main Flutter repo and that repo passes the analyzer, so I guess this is fixed.