require.js seems to be executing scripts out of order

a.js

define([], function() {
  console.log("loading a");
  return "module a";
})

b.js

define([], function() {
  console.log("loading b");
  return "module b";
})

base.html

<script src="require.js"></script>
<script>
require(["main"], function() {
  console.log("this should run after main, a, and b are loaded");
});
</script>

main.js

require(["a", "b"], function() {
  console.log("loading main");
});

I would expect the log lines to print in this order:

loading a
loading b
loading main
this should run after main, a, and b are loaded

But I'm seeing behavior more like:

this should run after main, a, and b are loaded
loading a
loading b
loading main