Trim any text before osascript JSON response (#166296)

Fixes https://github.com/flutter/flutter/issues/164772.

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packages/flutter_tools/lib/src/ios/xcode_debug.dart

@@ -269,8 +269,19 @@ class XcodeDebug {
 
   @visibleForTesting
   XcodeAutomationScriptResponse? parseScriptResponse(String results) {
+    // Some users reported text before the json. Trim any text before the opening
+    // curly brace.
+    // Example: `start process_extensions{"status":true,"errorMessage":null,"debugResult":{"completed":false,"status":"running","errorMessage":null}}`
+    final String trimmedResults;
+    final int jsonBeginIndex = results.indexOf('{');
+    if (jsonBeginIndex > -1) {
+      trimmedResults = results.substring(jsonBeginIndex);
+    } else {
+      trimmedResults = results;
+    }
+
     try {
-      final Object decodeResult = json.decode(results) as Object;
+      final Object decodeResult = json.decode(trimmedResults) as Object;
       if (decodeResult is Map<String, Object?>) {
         final XcodeAutomationScriptResponse response = XcodeAutomationScriptResponse.fromJson(
           decodeResult,
@@ -280,10 +291,10 @@ class XcodeDebug {
           return response;
         }
       }
-      _logger.printError('osascript returned unexpected JSON response: $results');
+      _logger.printError('osascript returned unexpected JSON response: $trimmedResults');
       return null;
     } on FormatException {
-      _logger.printError('osascript returned non-JSON response: $results');
+      _logger.printError('osascript returned non-JSON response: $trimmedResults');
       return null;
     }
   }

packages/flutter_tools/test/general.shard/ios/xcode_debug_test.dart

@@ -617,6 +617,27 @@ void main() {
         expect(logger.errorText, contains('osascript returned unexpected JSON response'));
         expect(response, isNull);
       });
+
+      testWithoutContext('successfully removes any text before JSON', () async {
+        final Xcode xcode = setupXcode(
+          fakeProcessManager: FakeProcessManager.any(),
+          fileSystem: fileSystem,
+          flutterRoot: flutterRoot,
+        );
+        final XcodeDebug xcodeDebug = XcodeDebug(
+          logger: logger,
+          processManager: fakeProcessManager,
+          xcode: xcode,
+          fileSystem: fileSystem,
+        );
+
+        final XcodeAutomationScriptResponse? response = xcodeDebug.parseScriptResponse(
+          'start process_extensions{"status":true,"errorMessage":null,"debugResult":{"completed":false,"status":"running","errorMessage":null}}',
+        );
+
+        expect(logger.errorText, isEmpty);
+        expect(response, isNotNull);
+      });
     });
 
     group('exit', () {