Loading icon is not showing for slow tree nodes

[IllusionMH]

Issue Type: Bug

In latest Insiders build instead of loading icon I see spinning chevron for some tree nodes.
It's mostly noticeable in GitHub Pull Request extension tree, however I was able to reproduce it in Call Hierarchy while expanding some nested trees.

Looks like .codicon-loading:before has lower priority than .codicon-tree-item-expanded:before which prevents icon change.

/cc @aeschli

VS Code version: Code - Insiders 1.45.0-insider (9328b32, 2020-04-22T08:44:55.510Z)
OS version: Windows_NT x64 10.0.18363

System Info

Item	Value
CPUs	Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz (12 x 3192)
GPU Status	2d_canvas: unavailable_software flash_3d: disabled_software flash_stage3d: disabled_software flash_stage3d_baseline: disabled_software gpu_compositing: disabled_software multiple_raster_threads: enabled_on oop_rasterization: disabled_off protected_video_decode: disabled_off rasterization: disabled_software skia_renderer: disabled_off_ok video_decode: disabled_software viz_display_compositor: enabled_on viz_hit_test_surface_layer: disabled_off_ok webgl: unavailable_software webgl2: unavailable_software
Load (avg)	undefined
Memory (System)	31.72GB (16.46GB free)
Process Argv
Screen Reader	no
VM	0%

Extensions: none