Suggestion: add plugin before other plugins

[danini-the-panini]

I've come across errors resembling the following:

08 05 2016 22:47:59.218:ERROR [coverage]: TypeError: Cannot read property 'text' of undefined
    at PROJECT_ROOT/node_modules/istanbul/lib/report/html.js:241:45

I have found that placing the istanbul plugin before the other plugins fixes the issue.

Causes Error:

// karma.conf.js
//...
    rollupPreprocessor: {
      rollup: {
        plugins: [
          // ... other plugins
          require("rollup-plugin-babel")(),
          require("rollup-plugin-istanbul")({
            exclude: ["test/**/*.js"]
          })
        ]
      }
    },
//...

Fixes Error:

// karma.conf.js
//...
    rollupPreprocessor: {
      rollup: {
        plugins: [
          require("rollup-plugin-istanbul")({
            exclude: ["test/**/*.js"]
          }),
          // ... other plugins
          require("rollup-plugin-babel")()
        ]
      }
    },
//...

@artberri Perhaps, if you think this is a good idea, the README ought to updated to reflect this?

[artberri]

I've merged your PR #5 , so, the readme now reflects it. Thank you and sorry for being so late.

[jlmakes]

I believe this is not fixed, and instead has created another error.

I don't have more than these 2 plugins, so I can speak to their order relative to others... but I have found that having rollup-plugin-istanbul before rollup-plugin-buble causes this error:

29 12 2016 04:13:15.760:ERROR [preprocessor.rollup]:
Failed to process /Users/jlmakes/development/project/test/utils/generic.spec.js

Unexpected token (12:3441) in /Users/jlmakes/development/project/src/utils/generic.js

The error reported by @jellymann appears to be an issue with Istanbul's poor handling of source maps (perhaps in conjunction with babel); see the discussion here:

karma-runner/karma-coverage#157
karma-runner/karma-coverage#167

And in the Istanbul repo:
gotwarlost/istanbul#429

There's even a fork with an alleged fix:
https://github.com/jansoren/istanbul