Stop serving Observatory by default (#122419)

Observatory can still be enabled by providing `--serve-observatory` or
invoking the `_serveObservatory` private service RPC via web socket or
HTTP.

Related to https://github.com/dart-lang/sdk/issues/50233

packages/flutter_tools/lib/src/device.dart

@@ -937,7 +937,7 @@ class DebuggingOptions {
     this.nativeNullAssertions = false,
     this.enableImpeller = ImpellerStatus.platformDefault,
     this.uninstallFirst = false,
-    this.serveObservatory = true,
+    this.serveObservatory = false,
     this.enableDartProfiling = true,
     this.enableEmbedderApi = false,
    }) : debuggingEnabled = true;

packages/flutter_tools/lib/src/runner/flutter_command.dart

@@ -474,7 +474,6 @@ abstract class FlutterCommand extends Command<void> {
   void addServeObservatoryOptions({required bool verboseHelp}) {
     argParser.addFlag('serve-observatory',
       hide: !verboseHelp,
-      defaultsTo: true,
       help: 'Serve the legacy Observatory developer tooling through the VM service.',
     );
   }

packages/flutter_tools/test/integration.shard/flutter_attach_test.dart

@@ -197,24 +197,22 @@ void main() {
     testWithoutContext('enables Observatory on run', () async {
         await flutterRun.run(
           withDebugger: true,
-          // TODO(bkonyi): uncomment once Observatory is disabled by default
-          // See https://github.com/dart-lang/sdk/issues/50233
-          // serveObservatory: true,
+          serveObservatory: true,
         );
         expect(await isObservatoryAvailable(), true);
     });
 
     testWithoutContext('enables Observatory on attach', () async {
-      await flutterRun.run(withDebugger: true, serveObservatory: false);
+      await flutterRun.run(withDebugger: true);
       // Bail out if Observatory is still served by default in the VM.
+      // TODO(bkonyi): replace this with the following once Observatory is disabled in the VM:
+      // expect(await isObservatoryAvailable(), isFalse);
       if (await isObservatoryAvailable()) {
         return;
       }
       await flutterAttach.attach(
         flutterRun.vmServicePort!,
-        // TODO(bkonyi): uncomment once Observatory is disabled by default
-        // See https://github.com/dart-lang/sdk/issues/50233
-        // serveObservatory: true,
+        serveObservatory: true,
       );
       expect(await isObservatoryAvailable(), true);
     });

packages/flutter_tools/test/integration.shard/test_driver.dart

@@ -505,7 +505,7 @@ class FlutterRunTestDriver extends FlutterTestDriver {
     bool expressionEvaluation = true,
     bool structuredErrors = false,
     bool singleWidgetReloads = false,
-    bool serveObservatory = true,
+    bool serveObservatory = false,
     String? script,
     List<String>? additionalCommandArgs,
   }) async {
@@ -545,7 +545,7 @@ class FlutterRunTestDriver extends FlutterTestDriver {
     bool startPaused = false,
     bool pauseOnExceptions = false,
     bool singleWidgetReloads = false,
-    bool serveObservatory = true,
+    bool serveObservatory = false,
     List<String>? additionalCommandArgs,
   }) async {
     _attachPort = port;