[Observatory] Switch to Dart 2.12 and remove redundant experiment

TEST=None
Change-Id: I16bf33a6fc78c7bdcd32b376919b348922777b47
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/176880
Commit-Queue: Ben Konyi <bkonyi@google.com>
Reviewed-by: Ben Konyi <bkonyi@google.com>

runtime/observatory/BUILD.gn

@@ -37,7 +37,6 @@ prebuilt_dart2js_action("build_observatory") {
     "-o",
     rebase_path(output),
     "--packages=" + rebase_path("../../.packages"),
-    "--enable-experiment=non-nullable",
     "--no-sound-null-safety",
   ]
   if (is_debug) {

runtime/observatory/analysis_options.yaml

@@ -1,7 +1,4 @@
 analyzer:
-  enable-experiment:
-    - non-nullable
-
   errors:
     dead_code: info
     getter_not_subtype_setter_types: info

runtime/observatory/pubspec.yaml

@@ -1,6 +1,6 @@
 name: observatory
 environment:
-  sdk: '>=2.10.0 <3.0.0'
+  sdk: '>=2.12.0-0 <3.0.0'
 
 dependencies:
   usage: 'any'