TypeError: Cannot read property 'text' of undefined

not entirely sure if this is an issue with istanbul directly or karma-coverage, but i think it's an istanbul thing so i created [this issue](https://github.com/gotwarlost/istanbul/issues/846) with them but you may want to look into it too:

```js
04 10 2017 09:34:48.018:ERROR [coverage]: TypeError: Cannot read property 'text' of undefined
at project/node_modules/istanbul/lib/report/html.js:288:53
at Array.forEach (native)
at annotateBranches (project/node_modules/istanbul/lib/report/html.js:255:30)
at HtmlReport.writeDetailPage (project/node_modules/istanbul/lib/report/html.js:426:9)
at project/node_modules/istanbul/lib/report/html.js:489:26
at SyncFileWriter.writeFile (project/node_modules/istanbul/lib/util/file-writer.js:57:9)
at FileWriter.writeFile (project/node_modules/istanbul/lib/util/file-writer.js:147:23)
at project/node_modules/istanbul/lib/report/html.js:488:24
at Array.forEach (native)
at HtmlReport.writeFiles (project/node_modules/istanbul/lib/report/html.js:482:23)
at project/node_modules/istanbul/lib/report/html.js:484:22
at Array.forEach (native)
at HtmlReport.writeFiles (project/node_modules/istanbul/lib/report/html.js:482:23)
at HtmlReport.writeReport (project/node_modules/istanbul/lib/report/html.js:566:14)
at writeReport (project/node_modules/karma-coverage/lib/reporter.js:68:16)
at project/node_modules/karma-coverage/lib/reporter.js:296:11
```

the problem is:

```
text = structuredText[startLine].text;
```
i guess it assumes structuredText[startLine] is always going to be a thing, and apparently at least once it is not a thing?
I don't know how you can reproduce it; just let me know what you need that i can provide and i'll provide it.

Start with the stack trace in istanbul/lib/report/html.js, especially annotateBranches at line 288, and inspect how structuredText and startLine are used there. Reproduce the failure with karma-coverage if possible; done means the coverage report no longer throws when structuredText[startLine] is unavailable.

Written by the indexing model from the issue text.