Added this to the Later milestone.
Removed Priority-Unassigned label.
Added Priority-Medium, Analyzer-Hint labels.

Removed this from the Later milestone.
Added Oldschool-Milestone-Later label.

Removed Oldschool-Milestone-Later label.

An alternative (and more general) fix would be to generate a hint when a return value of type Future is ignored by the caller.

In addition to catching the issue documented above, it would have caught the bug fixed by revision 40637.

The challenge would be to find an easy way to allow the user to suppress the hint in the (presumably rare) case where ignoring the future really is the correct thing to do.

This comment was originally written by @zoechi

Thanks for creating this issue.
This is a real pain point and I stumble upon this regularly myself and I think it is the most common Dart question on StackOverflow (why doesn't do this code what I expect).

This has been fixed. The analyzer now reports errors for the above example:

$ dartanalyzer --version
dartanalyzer version 2.0.0-dev.63.0
$ cat 18531.dart 
import 'dart:async';

Future foo(int i) {
  return new Future(() {});
}

Future<int> bar() {
  return foo(1).then(() {
    /* return */ foo(2).then(() {
      return 5;
    });
  });
}
$ dartanalyzer 18531.dart 
Analyzing 18531.dart...
  error • The argument type '() → Null' can't be assigned to the parameter type '(dynamic) → FutureOr<int>' at 18531.dart:8:22 • argument_type_not_assignable
  error • The argument type '() → int' can't be assigned to the parameter type '(dynamic) → FutureOr<dynamic>' at 18531.dart:9:30 • argument_type_not_assignable
2 errors found.