Only compiling files using ext once despite changes.

```json
grunt.config.merge({
babel: {
options: {
sourceMap: false,
presets: ['env']
},
dist: {
files: [{
expand: true,
cwd: '<%= env.DIR_SRC %>/assets/scripts/',
src: '**/*.js',
ext: '.cmp.js',
dest: '<%= env.DIR_SRC %>/assets/scripts/'
}]
}
},
```
This will compile the .cmp.js files, but any changes to the original js files will yeild no change until the .cmp.js files are deleted.

Start from the Grunt Babel configuration shown in the issue and inspect how existing .cmp.js outputs are handled when source .js files change. Reproduce the behavior with the listed expand, ext, and dest settings; done means recompiling reflects source changes without manually deleting the generated files.

Written by the indexing model from the issue text.