I found this strange bug in dart2js warnings. Consider the following code:
 
main() {
  int x = compute(3);
  m1(x);
  m2(x);
  m3(x);
  m4(x);
}

void m4(int d) {
  switch(d) {
    default:
      if (d > 10) {
        throw "foo";
      } else {
        return;
      }
      d++;
  }
}

void m3(int c) {
  if (c > 10) {
    throw "foo";
  } else {
    return;
  }
  c++;
}

void m2(int b) {
  switch(b) {
    default:
      throw "a";
      b++;
  }
}

void m1(int a) {
  throw "foo";
  a++;
}

int compute(y) => y == 0 ? y : y * compute(y - 1);

1. Calling dart2js on this program produces:
   program:warning.dart:17:7: Warning: unreachable code
         d++;
         ^^^^
   warning.dart:27:3: Warning: unreachable code
     c++;
     ^^^^
   warning.dart:34:7: Warning: unreachable code
         b++;
         ^^^^
   warning.dart:40:3: Warning: unreachable code
     a++;
     ^^^^
   Dart file warning.dart compiled to JavaScript: out.js

This looks great!

2. Comment out the calls to m1, m2 and m3, then the output becomes:
   warning.dart:17:7: Warning: unreachable code
         d++;
         ^^^^
   warning.dart:17:7: Warning: dead code
         d++;
         ^^^^
   Dart file warning.dart compiled to JavaScript: out.js

Strangely: the error on d++ is reported twice, and the second time uses a different error message.