Using require.ensure like so seems to work fine, and d3 gets split into a separate bundle file:

    Router.prototype.someMethod = function() {};

    Router.prototype.foo = function() {
      require.ensure([], function() {
        require("d3");
        return this.someMethod();
      });
    };

However, that call to this.someMethod() will fail, so I changed it to return a closure (switched -> to => in coffeescript):

    Router.prototype.foo = function() {
      require.ensure([], (function(_this) {
        return function() {
          require("d3");
          return _this.someMethod();
        };
      })(this));
    };

This change seems to break the code splitting, producing a single bundle.js file.

I've worked around the issue for now by using Function.prototype.bind() instead of the closure, but the current behavior was very surprising and took me a while to track down, so it seems worthwhile to fix so others don't run into it as well.