Check the realm file in its own shard. (#137160)

This way, it is easier to differentiate between a real failure in the tools tests vs. just the realm check that blocks the merge.

.ci.yaml

@@ -1099,6 +1099,21 @@ targets:
       - bin/**
       - .ci.yaml
 
+  - name: Linux realm_checker
+    bringup: true
+    recipe: flutter/flutter_drone
+    timeout: 60
+    properties:
+      add_recipes_cq: "true"
+      shard: realm_checker
+      tags: >
+        ["framework", "hostonly", "shard", "linux"]
+    runIf:
+      - dev/**
+      - packages/flutter_tools/**
+      - bin/**
+      - .ci.yaml
+
   - name: Linux web_benchmarks_canvaskit
     recipe: devicelab/devicelab_drone
     presubmit: false

TESTOWNERS

@@ -340,3 +340,4 @@
 # web_long_running_tests @yjbanov @flutter/web
 # web_tests @yjbanov @flutter/web
 # web_tool_tests @eliasyishak @flutter/tool
+# realm_checker @jacksongardner @flutter/tool

dev/bots/test.dart

@@ -262,6 +262,7 @@ Future<void> main(List<String> args) async {
       'web_long_running_tests': _runWebLongRunningTests,
       'flutter_plugins': _runFlutterPackagesTests,
       'skp_generator': _runSkpGeneratorTests,
+      'realm_checker': _runRealmCheckerTest,
       kTestHarnessShardName: _runTestHarnessTests, // Used for testing this script; also run as part of SHARD=framework_tests, SUBSHARD=misc.
     });
   } catch (error, stackTrace) {
@@ -1568,6 +1569,13 @@ Future<void> _runSkpGeneratorTests() async {
   );
 }
 
+Future<void> _runRealmCheckerTest() async {
+  final String engineRealm = File(engineRealmFile).readAsStringSync().trim();
+  if (engineRealm.isNotEmpty) {
+    foundError(<String>['The checked-in engine.realm file must be empty.']);
+  }
+}
+
 // The `chromedriver` process created by this test.
 //
 // If an existing chromedriver is already available on port 4444, the existing