Unreachable code not well detected

Flow version: 0.44.1
```
function foo(): string {
if (true) {
return ''
}
}

function bar(): string {
while (true) {
return ''
}
}
```
Flow complained:
```
1: function foo(): string {
^ string. This type is incompatible with an implicitly-returned undefined.
7: function bar(): string {
^ string. This type is incompatible with an implicitly-returned undefined.
```
while I expected it would pass since both implicitly-returned undefineds are unreachable.