Fix issue with problemMatcher when customising tasks
Fix issue with problemMatcher when customising tasks

See https://github.com/dart-lang/build/issues/1025#issuecomment-435993458 and https://github.com/Dart-Code/Dart-Code/issues/1211#issuecomment-470621513.

Dart-Code/Dart-Code,Dart-Code/Dart-Code,Dart-Code/Dart-Code,Dart-Code/Dart-Code,Dart-Code/Dart-Code