Example files should not be counted in test coverage requirements #459

Here I am getting a false positive because I'm adding examples: dart-lang/core#999

IMO, test coverage should only be counted for files in `lib/´.

When it comes to example/ subfolder, I agree that it would be nice to test examples. But really it's probably acceptable that they are covered by dart analyze.

Otherwise, we have to come up with a reasonable way to test them. Either, by saying you always make example/<examplepkg1>/bin/myexample.dart because then you can put tests for examples in to example/<examplepkg1>/test/ but probably the test setup we have in firehose and codecoverage doesn't do that. And for cases where we just want a single simple test file, it's total overkill to make a subfolder in example with pubspec and everything.

For now maybe the best approach is to not count files in example/ when collecting code coverage.

Same with files in tool/, etc.. probably we should only consider files in lib/.