Roll engine a1a2129bc469..86f4b741b863 (5 commits) (#30223)

https://github.com/flutter/engine/compare/a1a2129bc469...86f4b741b863

git log a1a2129bc4696e66510be984fd7908eae51eb98f..86f4b741b863aef5586f03f4d250f1d789fde2be --no-merges --oneline
86f4b741b Move libdart selection into its own target in //flutter/runtime. (flutter/engine#8373)
f39cf3774 Add a GN format presubmit. (flutter/engine#8371)
d6e2d8527 Roll src/third_party/skia 7eeb74fdfd84..73c6c9d26870 (8 commits) (flutter/engine#8372)
8412c199a Enable Linux shell build (flutter/engine#8233)
133901199 libtxt: track the start and end x positions of glyph blobs for more accurate rendering of text decorations (flutter/engine#8368)

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bin/internal/engine.version

-a1a2129bc4696e66510be984fd7908eae51eb98f
+86f4b741b863aef5586f03f4d250f1d789fde2be