Dartanalyzer allows fields override in a strong mode for mixins

[sgrekhov]

In a strong mode it is prohibited to override fields. Therefore Analyzer in a strong mode reports an error for the following code

class A {
  int x = 0;
}
class M {
  int x = 1;
}
class C extends A with M {
}
main() {
}

Analyzer output in this case is

[error] Field declaration A.x cannot be overridden in M.

But it doesn't report any issues for the following code

class A {
  int x = 0;
}
class M {
  int x = 1;
}
class C = A with M;
main() {
}

It should be the same error in the case above.
Tested on dartanalyzer version 1.20.0-dev.7.0

[bwilkerson]

My understanding is that strong mode is going to be relaxed to allow field overriding, so this might become a non-issue.

[jmesserly]

My understanding is that strong mode is going to be relaxed to allow field overriding, so this might become a non-issue.

whatever the default, we still need something to indicate fields are sealed, though. So this bug is important to get fixed.

[jmesserly]

merging this into #27502 (comment)

[jmesserly]

Digging deeper, this appears more serious than I thought and affects much more than fields. I've filed: #29426