Import maps are not updated in incremental reload for preview

[cinxmo]

Steps to reproduce:

1. Create a new page blah.md in docs and open http://127.0.0.1:3000/blah.
2. In a js cell, type Inputs.table(<some data>):

```js
display(Inputs.table([{a: 1, b: 2}]));
```

3. In the browser, notice an error message that says "cannot dynamically load..."
4. Refresh the page and the error message goes away

Import maps should be updated without having to refresh the page

[cinxmo]

@Fil does this ticket capture the steps correctly? Thanks!

[Fil]

No need to delete anything:

1. Create a new page blah.md in docs/, type # Blah, save, open http://127.0.0.1:3000/blah
2. type

```js
display(Inputs.table([{a: 1, b: 2}]));
```

the browser updates its page with the error message; the import map is incomplete. (Reload, the message goes away.)

[mootari]

Related: #14

[mbostock]

Duplicate of #14.