Roll engine 2586e9412257..a6e816b2a67c (14 commits) (#23513)

https://github.com/flutter/engine/compare/2586e9412257...a6e816b2a67c

git log 2586e9412257..a6e816b2a67c --no-merges --oneline
a6e816b2a Roll src/third_party/skia 4f598e8c829a..dfca8f6adb6b (7 commits) (flutter/engine#6658)
ff0525f59 Add missing entry-points. (flutter/engine#6634)
08771fe14 Revert dart before engine commit 26e8840de5d6fe801378301dd36d54c92b25bd3b (flutter/engine#6656)
914551bfa Dart SDK roll for 2018-10-24
9f2e2ba7b Add/expose API for Paragraph.getBoxesForRange BoxHeightStyle and BoxWidthStyle. (flutter/engine#6644)
9669b70e1 Add an iOS PlatformViewsController for creating/disposing UIViews. (flutter/engine#6569)
6c2477bd2 fix setter for viewOpaque (flutter/engine#6653)
2bd8c94fc Roll src/third_party/skia a7682c8c73e4..4f598e8c829a (8 commits) (flutter/engine#6654)
e1e60933b Realize kernel asset mappings on a worker thread if one is available. (flutter/engine#6648)
d3874a9ca Roll src/third_party/skia 3b57279fd65a..a7682c8c73e4 (9 commits) (flutter/engine#6652)
3dddf4d5b Roll src/third_party/skia 8429c7930291..3b57279fd65a (2 commits) (flutter/engine#6650)
7b3bb62a8 Roll src/third_party/skia 3aca39df7b22..8429c7930291 (5 commits) (flutter/engine#6647)
26e8840de Dart SDK roll for 2018-10-23
9829c51a9 Roll src/third_party/skia b46c4d0925ad..3aca39df7b22 (12 commits) (flutter/engine#6643)


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bin/internal/engine.version

-2586e941225726f0429b9e67f4d9b6ab404d7b1e
+a6e816b2a67c9087d81345da3fab87077bc45c91