See also https://groups.google.com/a/dartlang.org/forum/?fromgroups=#!topic/compiler-dev/SOcapH1LPPc for discussion.

One suggestion is to rewrite to:

import 'dart:html';

class Looper {
  double _renderTime = 0; // A field

  var loop; // Cached closure to pass to rAF.

  Looper() {
    loop = _loop; // Constructor needed only to initialize the cached closure.
  }

  void kick() {
    window.requestAnimationFrame(loop);
  }

  _loop(currentTime) {
    kick();
    if (currentTime is !num) throw "Impossible"; // Teach that currentTime is num.

    if (_renderTime == 0)
      _renderTime = currentTime;

    if (_renderTime > currentTime)
      _renderTime = currentTime;

    double deltaTime = currentTime - _renderTime;
    double deltaTimeSec = deltaTime / 1000.0;
    double currentTimeSec = currentTime / 1000.0;

    if (deltaTime >= 1) {
      _renderTime = currentTime;
    }
  }
}

void main() {
  new Looper().kick();
}

Added C3 label.

Added this to the Later milestone.

Added TriageForM5 label.

Removed TriageForM5 label.

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

Removed Oldschool-Milestone-Later label.

I think this is fixed, unless I'm running dart2js differently. Lines reduced from 21 to 16:

$ cat issue-10071.dart
import 'dart:html';

double _renderTime = 0;

loop(final num currentTime) {
  window.requestAnimationFrame(loop);

  if (_renderTime == 0)
    _renderTime = currentTime;

  if (_renderTime > currentTime)
    _renderTime = currentTime;

  double deltaTime = currentTime - _renderTime;
  double deltaTimeSec = deltaTime / 1000.0;
  double currentTimeSec = currentTime / 1000.0;

  if (deltaTime >= 1) {
    _renderTime = currentTime;
  }
}

void main() {
  window.requestAnimationFrame(loop);
}

$ dart2js issue-10071.dart
issue-10071.dart:3:20:
Warning: 'int' is not assignable to 'double'.
double _renderTime = 0;
                   ^
Dart file (issue-10071.dart) compiled to JavaScript: out.js

$ grep -A 16 'loop:' out.js
  loop: [function(currentTime) {
    var t1, deltaTime;
    t1 = window;
    C.Window_methods._ensureRequestAnimationFrame$0(t1);
    C.Window_methods._requestAnimationFrame$1(t1, W._wrapZone(F.loop$closure()));
    if (J.$eq($._renderTime, 0))
      $._renderTime = currentTime;
    if (J.$gt$n($._renderTime, currentTime))
      $._renderTime = currentTime;
    deltaTime = J.$sub$n(currentTime, $._renderTime);
    if (typeof deltaTime !== "number")
      return deltaTime.$div();
    if (typeof currentTime !== "number")
      return currentTime.$div();
    if (deltaTime >= 1)
      $._renderTime = currentTime;
  }, "call$1", "loop$closure", 2, 0, 6],

Only 2 check/bailout pairs, down from 4.

But it uses non-typed interceptor methods.
It looks like we don't catch that requestAnimationFrame calls the function with a double.