Parse comments in head

Unit tests on `web_idl_files_test.dart` looking better now after this PR: `46 PASSED, 0 FAILED, 11 ERRORS`

```
unittest-suite-wait-for-done
PASS: Test valid web idl files Testing test/idl/valid/allowany.widl
PASS: Test valid web idl files Testing test/idl/valid/array.widl
PASS: Test valid web idl files Testing test/idl/valid/attributes.widl
PASS: Test valid web idl files Testing test/idl/valid/callback.widl
PASS: Test valid web idl files Testing test/idl/valid/caller.widl
ERROR: Test valid web idl files Testing test/idl/valid/constants.widl
  Test failed: Caught line 10, character 22: expected '-', 'null', 'Infinity', float, integer, 'true', 'false' or 'NaN', got '.'.
PASS: Test valid web idl files Testing test/idl/valid/constructor.widl
PASS: Test valid web idl files Testing test/idl/valid/dictionary-inherits.widl
PASS: Test valid web idl files Testing test/idl/valid/dictionary.widl
PASS: Test valid web idl files Testing test/idl/valid/documentation-dos.widl
PASS: Test valid web idl files Testing test/idl/valid/documentation.widl
PASS: Test valid web idl files Testing test/idl/valid/enum.widl
PASS: Test valid web idl files Testing test/idl/valid/equivalent-decl.widl
PASS: Test valid web idl files Testing test/idl/valid/exception-inheritance.widl
PASS: Test valid web idl files Testing test/idl/valid/exception.widl
PASS: Test valid web idl files Testing test/idl/valid/getter-setter.widl
PASS: Test valid web idl files Testing test/idl/valid/identifier-qualified-names.widl
PASS: Test valid web idl files Testing test/idl/valid/implements.widl
PASS: Test valid web idl files Testing test/idl/valid/indexed-properties.widl
PASS: Test valid web idl files Testing test/idl/valid/inherits-getter.widl
PASS: Test valid web idl files Testing test/idl/valid/interface-inherits.widl
ERROR: Test valid web idl files Testing test/idl/valid/iterator.widl
  Test failed: Caught line 5, character 19: expected '(', got ';'.
PASS: Test valid web idl files Testing test/idl/valid/mapclass.widl
ERROR: Test valid web idl files Testing test/idl/valid/namedconstructor.widl
  Test failed: Caught line 2, character 19: unexpected 'A'.
PASS: Test valid web idl files Testing test/idl/valid/nointerfaceobject.widl
PASS: Test valid web idl files Testing test/idl/valid/nullable.widl
PASS: Test valid web idl files Testing test/idl/valid/nullableobjects.widl
PASS: Test valid web idl files Testing test/idl/valid/operation-optional-arg.widl
PASS: Test valid web idl files Testing test/idl/valid/overloading.widl
PASS: Test valid web idl files Testing test/idl/valid/overridebuiltins.widl
PASS: Test valid web idl files Testing test/idl/valid/partial-interface.widl
PASS: Test valid web idl files Testing test/idl/valid/primitives.widl
PASS: Test valid web idl files Testing test/idl/valid/prototyperoot.widl
ERROR: Test valid web idl files Testing test/idl/valid/putforwards.widl
  Test failed: Caught line 3, character 16: unexpected 'f'.
PASS: Test valid web idl files Testing test/idl/valid/reg-operations.widl
PASS: Test valid web idl files Testing test/idl/valid/replaceable.widl
ERROR: Test valid web idl files Testing test/idl/valid/sequence.widl
  Test failed: Caught line 4, character 29: unexpected 'f'.
ERROR: Test valid web idl files Testing test/idl/valid/serializer.widl
  Test failed: Caught line 8, character 13: expected identifier, '?', '(', '[' or spaces, got ';'.
ERROR: Test valid web idl files Testing test/idl/valid/static.widl
  Test failed: Caught line 9, character 10: expected 'DOMString', 'byte', 'Date', 'octet', identifier, 'double', 'unrestricted', 'any', 'object', 'boolean', 'short', 'void', 'float', 'sequence', 'long', 'unsigned' or '(', got 'r'.
PASS: Test valid web idl files Testing test/idl/valid/stringifier-attribute.widl
PASS: Test valid web idl files Testing test/idl/valid/stringifier-custom.widl
PASS: Test valid web idl files Testing test/idl/valid/stringifier.widl
ERROR: Test valid web idl files Testing test/idl/valid/treatasnull.widl
  Test failed: Caught line 6, character 40: unexpected 'E'.
ERROR: Test valid web idl files Testing test/idl/valid/treatasundefined.widl
  Test failed: Caught line 6, character 45: unexpected 'E'.
ERROR: Test valid web idl files Testing test/idl/valid/typedef.widl
  Test failed: Caught line 7, character 24: unexpected 'P'.
ERROR: Test valid web idl files Testing test/idl/valid/typesuffixes.widl
  Test failed: Caught line 2, character 22: unexpected 'D'.
PASS: Test valid web idl files Testing test/idl/valid/uniontype.widl
PASS: Test valid web idl files Testing test/idl/valid/variadic-operations.widl
PASS: Test invalid web idl files Testing test/idl/invalid/enum.widl
PASS: Test invalid web idl files Testing test/idl/invalid/module.widl
PASS: Test invalid web idl files Testing test/idl/invalid/nonnullableany.widl
PASS: Test invalid web idl files Testing test/idl/invalid/nonnullableobjects.widl
PASS: Test invalid web idl files Testing test/idl/invalid/raises.widl
PASS: Test invalid web idl files Testing test/idl/invalid/scopedname.widl
PASS: Test invalid web idl files Testing test/idl/invalid/sequenceAsAttribute.widl
PASS: Test invalid web idl files Testing test/idl/invalid/special-omittable.widl
PASS: Test invalid web idl files Testing test/idl/invalid/stringconstants.idl

46 PASSED, 0 FAILED, 11 ERRORS
Unhandled exception:
Exception: Some tests failed.
#0      SimpleConfiguration.onDone (package:unittest/src/simple_configuration.dart:213:9)
#1      _completeTests (package:unittest/unittest.dart:795:17)
#2      _runTest (package:unittest/unittest.dart:751:19)
#3      _nextTestCase (package:unittest/unittest.dart:658:11)
#4      _asyncRunCallback (dart:async/event_loop.dart:9:15)
#5      _asyncRunCallback (dart:async/event_loop.dart:13:7)
#6      _createTimer.<anonymous closure> (dart:async-patch/timer_patch.dart:8:13)
#7      _Timer._createTimerHandler._handleTimeout (timer_impl.dart:95:21)
#8      _Timer._createTimerHandler._handleTimeout (timer_impl.dart:103:7)
#9      _Timer._createTimerHandler._handleTimeout (timer_impl.dart:103:7)
#10     _Timer._createTimerHandler.<anonymous closure> (timer_impl.dart:111:23)
#11     _ReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:81:92)
```