Cannot visualize lcov coverage data

The `flutter test --coverage` command produces an `coverage/lcov.info` file that contains information about the line coverage of your tests. We should be able to visualize that information in IntelliJ.

1. When editing a file, you should be able to see whether a line was (a) executed in a test, (b) possible to execute but not actually executed, (c) not actually possible to execute. (All this data is in the lcov file.)

2. There should be some sort of overview panel that lists the various files in the project and what percent line coverage they have. Also, there should be an overall readout for the percent coverage of the whole project.

For example, the [lcov-info](https://atom.io/packages/lcov-info) plugin does this for Atom. It's likely that such a plugin already exists for IntelliJ. We might simply need to find it and recommend.