[ package:vm_service ] Additional Observatory test migrations

Also bumped minimum SDK requirement to 3.0.0 to make use of new language
features.

Work towards https://github.com/dart-lang/sdk/issues/45037

Change-Id: I3543dd902a69f9490087f1f263aeb2dabb488fa0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/326841
Reviewed-by: Derek Xu <derekx@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

https://dart.googlesource.com/sdk/+/b68f533639ea0cb38fd09b8b7243bad1aa830e0c

DEPS

diff --git a/DEPS b/DEPS
index 1d03435..9611b69 100644
--- a/DEPS
+++ b/DEPS
@@ -267,7 +267,7 @@
 
   'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + '20458e1729988520b7be9e49c8972b9473396eb3',
 
-  'engine/src/third_party/dart': Var('dart_git') + '/sdk' + '@' + '990e595bf025e65917f8db771dff132363e949df',
+  'engine/src/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'b68f533639ea0cb38fd09b8b7243bad1aa830e0c',
 
   'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'afc391602b0682f16ae3f12e2d5c908b5a90df8b',
 

commits.json

diff --git a/commits.json b/commits.json
index f947910..858e049 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"20458e1729988520b7be9e49c8972b9473396eb3",
-  "engine/src/third_party/dart":"990e595bf025e65917f8db771dff132363e949df",
+  "engine/src/third_party/dart":"b68f533639ea0cb38fd09b8b7243bad1aa830e0c",
   "flutter":"afc391602b0682f16ae3f12e2d5c908b5a90df8b"
 }
\ No newline at end of file