`export default` in namespace crashes compiler

[romdotdog]

Code

namespace Foo {
  export default foo;
}

Try it in the playground

Stack trace

▌ Whoops, the AssemblyScript compiler has crashed during initialize :-(
▌ 
▌ Here is the stack trace hinting at the problem, perhaps it's useful?
▌ 
▌ AssertionError: assertion failed
▌     at assert (D:\assemblyscript\std\portable\index.js:200:11)
▌     at Program.initializeNamespace (D:\assemblyscript\src\program.ts:2545:18)
▌     at Program.initialize (D:\assemblyscript\src\program.ts:1114:18)
▌     at Object.initializeProgram (D:\assemblyscript\src\index.ts:274:11)
▌     at D:\assemblyscript\cli\asc.js:886:22
▌     at measure (D:\assemblyscript\cli\asc.js:1409:3)
▌     at Object.main (D:\assemblyscript\cli\asc.js:884:27)
▌     at runTest (D:\assemblyscript\tests\compiler.js:178:7)
▌     at Array.forEach (<anonymous>)
▌     at D:\assemblyscript\tests\compiler.js:557:16
▌     at processTicksAndRejections (node:internal/process/task_queues:96:5)
▌ 
▌ If it refers to the dist files, try to 'npm install source-map-support' and
▌ run again, which should then show the actual code location in the sources.
▌ 
▌ If you see where the error is, feel free to send us a pull request. If not,
▌ please let us know: https://github.com/AssemblyScript/assemblyscript/issues
▌ 
▌ Thank you!

What now?

There's a confusing error (in my opinion) that TypeScript generates in this case

A default export can only be used in an ECMAScript-style module.

I think a custom error for this case would be more informative than the above.