Heavily based on @jsanchezgarcia work in angular#4476.

Fix angular#3857

BREAKING CHANGE:

A new environmentSource entry replaces the previous source entry inside environments.

To migrate the code follow the example below:

Before:
```
"environments": {
  "source": "environments/environment.ts",
  "dev": "environments/environment.ts",
  "prod": "environments/environment.prod.ts"
}
```

After:
```
"environmentSource": "environments/environment.ts",
  "environments": {
  "dev": "environments/environment.ts",
  "prod": "environments/environment.prod.ts"
}
```