Ignore enhancements

[kturney]

Adds some enhancements to ignore comment handling:

1. allow omitting space between // and coverage in coverage ignore comments
2. allow text after coverage ignore comments
3. throw FormatException when encountering unbalanced ignore comments instead of silently erroring

Hopefully 1 and 2 are uncontroversial.
3 did change an existing test.
We found it quite confusing in our projects when seemingly ignored lines were still appearing as uncovered.
Only after much investigation would various devs discover that it was due to accidental nesting of ignore ranges.
To my mind, the coverage tool should lean towards being permissive (or maybe hard fail?).
Potentially an analyzer rule could be added to detect unbalanced coverage ignore comments.

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[liamappelbe]

We found it quite confusing in our projects when seemingly ignored lines were still appearing as uncovered.
Only after much investigation would various devs discover that it was due to accidental nesting of ignore ranges.
To my mind, the coverage tool should lean towards being permissive (or maybe hard fail?).

Sounds reasonable overall, but I actually lean towards a hard fail. You could just throw a FormatException with the file name and line number.

At the very least we should report a warning. This package doesn't really have any logging infrastructure yet though, so maybe just print a warning message to stderr. The cleaner solution would be to use package:logging, if you don't mind the extra effort.

[kturney]

@liamappelbe hard fail sounds good to me. Seems like it would be more obvious what the issue is in CI environments, especially with noisy logs.

I'll move forward with adding the FormatException unless I hear back that you'd prefer logging.

[kturney]

Changed to throwing FormatException with the file name and line number for unmatched coverage ignore comments

[coveralls]

coverage: 93.434% (+0.06%) from 93.379% when pulling 7c15965 on kturney:ignore-enhancements into 523cf4d on dart-lang:master.

[liamappelbe]

Looking good. Almost ready.

[liamappelbe]

The structure of this test is a bit odd. Can you refactor it like this:

test('throws FormatException when the annotations are not balanced', () {
  runTest(String source, String errMsg) {
    final content = invalidSources[i].split('\n');
    expect(...);
  }

  runTest(invalidSources[0], 'coverage:ignore-start found at content-0.dart:3'
                ' before previous coverage:ignore-start ended');
  runTest(invalidSources[1], 'coverage:ignore-start found at content-1.dart:3'
                ' before previous coverage:ignore-start ended');
  ...
});