13772 Move Path and PathMetrics from canvas.dart into their own files. No delta (cla: yes)

13779 [web] Fix path to svg for drrect (cla: yes)

13780 Allow passing hot reload debugging flags through (cla: yes)

13781 Create a WeakPtrFactory for use on the UI thread in VsyncWaiter (cla: yes)

13782 Document the coordinate space of points in FlutterPointerEvent. (cla: yes)

13784 Add Helvetica and sans-serif as fallback font families (cla: yes)

13785 Fix RendererContextSwitch result check in Rasterizer::MakeRasterSnapshot (cla: yes)

13786 Take devicePixelRatio into account when drawing shadows (cla: yes)