Using #warning in JS library files diagnoses three times.

`liba.js`
```js
#warning This is a test.
```

results in

```
$ emcc test/hello_world.c --js-library liba.js -o a.js

warning: liba.js:1: #warning This is a test.
warning: liba.js:1: #warning This is a test.
emcc: warning: warnings in JS library compilation [-Wjs-compiler]
```

First line is duplicated.

Is there value in the third line to be separate?

E.g. could this be

```
$ emcc test/hello_world.c --js-library liba.js -o a.js

warning: liba.js:1: #warning This is a test. [-Wjs-compiler]
```