Refactor a source for adding coverage

For example attaching method to increase coverage for a line to the window,
currently just creates a single observable

``` js
const coverage$ = Rx.Observable.create(function (observer) {
function incrementCoverage (line) {
if (fileCoverage.l[line] === undefined) {
console.error('there is no source on line', line)
return
}
fileCoverage.l[line] += 1
observer.onNext(fileCoverage)
}
window.incrementCoverage = incrementCoverage
}).startWith(fileCoverage)
```

Start with the source containing the Rx.Observable.create snippet and inspect how coverage$ and window.incrementCoverage are used. The issue provides no target file, test, or explicit completion criteria, so clarify whether the observable construction and coverage increment behavior should be separated before deciding what done means.

Written by the indexing model from the issue text.