At most 1 dep in depMap

Trace API only keeps the `last` imported module in `System.loads.*.depMap`. 

So for instance
```js
import a from './a.js'
import b from './b.js'
```

has a depMap of

```js
{ "./b.js" : "https://localhost:3000/app/b.js" }
```
 while 
```js
import b from './b.js'
import a from './a.js'
```
has a depMap of 
```js
{ "./a.js" : "https://localhost:3000/app/a.js" }
```