Allow trailing required positional parameters

[DartBot]

This issue was originally filed by @seaneagan

---

Problem: It's much more readable to pass callbacks as the last argument to a function call so as not to bury other args behind a long code block. But that's not currently possible if:

• the callback is required
• the function call also has optional args

since optional args must go after positional args.

Either of issue #4188 or issue #15398 would solve this problem for when the optional args are named.

But the following proposal (this issue) solves this problem for when the optionals are positional:

Allow trailing required positional parameters:

task(String name, [List<String> dependencies = const []], callback()) {
    // ...
}

task('init', () {
    // Long
    // code
    // block.
});

task('build', ['init'], () {
    // Long
    // code
    // block.
});

The best we can currently do is:

task(String name, depsOrCallback, [callback()]) {
    var deps = callback == null ? const [] : depsOrCallback;
    if (callback == null) callback = depsOrCallback;
    // ...
}

which:

• Loses the type (and thus documentation and checked-mode safety)
• Loses the default value (and thus documentation and auto-initialization)
• Requires extra logic to assign the parameters correctly.

[floitschG]

Added Area-Language, Triaged labels.

[munificent]

Duplicate of #15398.