[analysis_server] Allow allowOverlappingHandlers to be controlled via initializationOptions

For troubleshooting, this allows the client to override this flag.

I'd originally planned to use ClientConfiguration for this and allow it to be changed on-the-fly, however this can result in a mix of requests and therefore I decided it best to only support setting once during startup (which is part of initialization, and therefore before general requests start being sent).

This will require some Dart-Code work to provide a value here (and without a value, it will always be the servers default).

See https://github.com/dart-lang/sdk/issues/60440

Change-Id: Ie9843543d6d491afb046f3d1106211b7db852605
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/419541
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Phil Quitslund <pquitslund@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Phil Quitslund <pquitslund@google.com>

https://dart.googlesource.com/sdk/+/21c05846318c6f307121c3f075a7552a4c41b4d4

DEPS

diff --git a/DEPS b/DEPS
index 98158c0..c4b9f07 100644
--- a/DEPS
+++ b/DEPS
@@ -238,7 +238,7 @@
 ]
 
 deps = {
-  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '0476f21604d807999a895f2fdec320be60bf6d0b',
+  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '21c05846318c6f307121c3f075a7552a4c41b4d4',
 
   'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'a0b1b3253416b1f19e3f105b596eb6e4c5813ef4',
 

commits.json

diff --git a/commits.json b/commits.json
index 73ddf40..2aedc92 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
   "flutter":"a0b1b3253416b1f19e3f105b596eb6e4c5813ef4",
-  "flutter/engine/src/flutter/third_party/dart":"0476f21604d807999a895f2fdec320be60bf6d0b"
+  "flutter/engine/src/flutter/third_party/dart":"21c05846318c6f307121c3f075a7552a4c41b4d4"
 }
\ No newline at end of file