use event.client for absolute position
move context menu up when cropped
Fixed return warnings and return futures or throw errors where necessary (switched _handleMessage and _sendResponse return types as they were mismatched).
Merge pull request #1068 from dart-lang/umop-services-fix_warnings

Fixed return warnings and return futures or throw errors where necessary.
restore getAbsolutePosition, getEventAbsolutePosition
Merge pull request #1064 from sunglim/context_position

Context position