I was doing a quick example, changed something from being a regular function to being a simple lambda, and forgot to take out the return statement. So I get a method that looks like
  toString() => return _foo.toString();
and the error message in the editor could be more informative. I get six problems.

A non-abstract method must have a body
Duplicate member 'toString'
Duplicate member 'toString'
Unexpected token '.'
Unexpected token 'IDENTIFIER' (expected ';')
Unexpected token 'return'

The last one is the most useful, but I'd think this is an easy beginner mistake, and we could do better identifying it.