performance_timeline library Null safety

Classes

LargestContentfulPaint
See https://github.com/WICG/LargestContentfulPaint and largest_contentful_paint.idl
LayoutShift
See https://wicg.github.io/layout-instability/#sec-layout-shift and layout_shift.idl
LayoutShiftAttribution
PerformanceTimelineApi
Reporting of performance timeline events, as specified in https://w3c.github.io/performance-timeline/#dom-performanceobserver.
TimelineEvent