[vm, compiler] Use load-acquire for accessing _HashVMImmutableBase.index.

TEST=tsan
Cq-Include-Trybots: luci.dart.try:vm-tsan-linux-release-x64-try,vm-tsan-linux-release-arm64-try,iso-stress-linux-arm64-try,iso-stress-linux-x64-try
Change-Id: I15f9af02a9e0353bd93b14483f7fd18f25023308
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/442504
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

https://dart.googlesource.com/sdk/+/dff5fe7f855876df9e5e12c8f009a5df92806016

DEPS

diff --git a/DEPS b/DEPS
index df2b3bb..9096f46 100644
--- a/DEPS
+++ b/DEPS
@@ -229,7 +229,7 @@
 ]
 
 deps = {
-  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'ad7f7860d9688ae3f537c45b8f95f292ef558a11',
+  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'dff5fe7f855876df9e5e12c8f009a5df92806016',
 
   'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '30ae030c625dd3b9852c7388c10fe996dc166192',
 

commits.json

diff --git a/commits.json b/commits.json
index 44624f0..921a383 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
   "flutter":"30ae030c625dd3b9852c7388c10fe996dc166192",
-  "flutter/engine/src/flutter/third_party/dart":"ad7f7860d9688ae3f537c45b8f95f292ef558a11"
+  "flutter/engine/src/flutter/third_party/dart":"dff5fe7f855876df9e5e12c8f009a5df92806016"
 }
\ No newline at end of file