Add additional logging when devtools cannot launch (#81554)

packages/flutter_tools/lib/src/devtools_launcher.dart

@@ -67,9 +67,16 @@ class DevtoolsServerLauncher extends DevtoolsLauncher {
         final io.HttpClientResponse response = await request.close();
         await response.drain<void>();
         if (response.statusCode != io.HttpStatus.ok) {
+          _logger.printTrace(
+            'Skipping devtools launch because pub.dev responded with HTTP '
+            'status code ${response.statusCode} instead of ${io.HttpStatus.ok}.',
+          );
           offline = true;
         }
-      } on Exception {
+      } on Exception catch (e) {
+        _logger.printTrace(
+          'Skipping devtools launch because connecting to pub.dev failed with $e',
+        );
         offline = true;
       } on ArgumentError {
         if (!useOverrideUrl) {

packages/flutter_tools/test/general.shard/devtools_launcher_test.dart

@@ -363,4 +363,48 @@ void main() {
 
     expect(logger.errorText, contains('Failed to launch DevTools: ProcessException'));
   });
+
+  testWithoutContext('DevtoolsLauncher prints trace if connecting to pub.dev throws', () async {
+    final DevtoolsLauncher launcher = DevtoolsServerLauncher(
+      pubExecutable: 'pub',
+      logger: logger,
+      platform: platform,
+      persistentToolState: persistentToolState,
+      httpClient: FakeHttpClient.list(<FakeRequest>[
+        FakeRequest(
+          Uri.https('pub.dev', ''),
+          method: HttpMethod.head,
+          responseError: Exception('Connection failed.'),
+        ),
+      ]),
+      processManager: FakeProcessManager.empty(),
+    );
+
+    await launcher.launch(Uri.parse('http://127.0.0.1:1234/abcdefg'));
+
+    expect(logger.traceText, contains('Skipping devtools launch because connecting to pub.dev failed with Exception: Connection failed.'));
+  });
+
+  testWithoutContext('DevtoolsLauncher prints trace if connecting to pub.dev returns non-OK status code', () async {
+    final DevtoolsLauncher launcher = DevtoolsServerLauncher(
+      pubExecutable: 'pub',
+      logger: logger,
+      platform: platform,
+      persistentToolState: persistentToolState,
+      httpClient: FakeHttpClient.list(<FakeRequest>[
+        FakeRequest(
+          Uri.https('pub.dev', ''),
+          method: HttpMethod.head,
+          response: const FakeResponse(
+            statusCode: HttpStatus.forbidden
+          ),
+        ),
+      ]),
+      processManager: FakeProcessManager.empty(),
+    );
+
+    await launcher.launch(Uri.parse('http://127.0.0.1:1234/abcdefg'));
+
+    expect(logger.traceText, contains('Skipping devtools launch because pub.dev responded with HTTP status code 403 instead of 200.'));
+  });
 }