diff --git a/packages/flutter_tools/bin/tool_backend.dart b/packages/flutter_tools/bin/tool_backend.dart
index 13ce668b3e..f8b278f35d 100644
--- a/packages/flutter_tools/bin/tool_backend.dart
+++ b/packages/flutter_tools/bin/tool_backend.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 // Do not add package imports to this file.
 import 'dart:convert'; // flutter_ignore: dart_convert_import.
 import 'dart:io'; // flutter_ignore: dart_io_import.
@@ -13,24 +11,33 @@ Future<void> main(List<String> arguments) async {
   final String targetPlatform = arguments[0];
   final String buildMode = arguments[1].toLowerCase();
 
-  final String dartDefines = Platform.environment['DART_DEFINES'];
+  final String? dartDefines = Platform.environment['DART_DEFINES'];
   final bool dartObfuscation = Platform.environment['DART_OBFUSCATION'] == 'true';
-  final String extraFrontEndOptions = Platform.environment['EXTRA_FRONT_END_OPTIONS'];
-  final String extraGenSnapshotOptions = Platform.environment['EXTRA_GEN_SNAPSHOT_OPTIONS'];
-  final String flutterEngine = Platform.environment['FLUTTER_ENGINE'];
-  final String flutterRoot = Platform.environment['FLUTTER_ROOT'];
+  final String? extraFrontEndOptions = Platform.environment['EXTRA_FRONT_END_OPTIONS'];
+  final String? extraGenSnapshotOptions = Platform.environment['EXTRA_GEN_SNAPSHOT_OPTIONS'];
+  final String? flutterEngine = Platform.environment['FLUTTER_ENGINE'];
+  final String? flutterRoot = Platform.environment['FLUTTER_ROOT'];
   final String flutterTarget = Platform.environment['FLUTTER_TARGET']
     ?? pathJoin(<String>['lib', 'main.dart']);
-  final String codeSizeDirectory = Platform.environment['CODE_SIZE_DIRECTORY'];
-  final String localEngine = Platform.environment['LOCAL_ENGINE'];
-  final String projectDirectory = Platform.environment['PROJECT_DIR'];
-  final String splitDebugInfo = Platform.environment['SPLIT_DEBUG_INFO'];
-  final String bundleSkSLPath = Platform.environment['BUNDLE_SKSL_PATH'];
+  final String? codeSizeDirectory = Platform.environment['CODE_SIZE_DIRECTORY'];
+  final String? localEngine = Platform.environment['LOCAL_ENGINE'];
+  final String? projectDirectory = Platform.environment['PROJECT_DIR'];
+  final String? splitDebugInfo = Platform.environment['SPLIT_DEBUG_INFO'];
+  final String? bundleSkSLPath = Platform.environment['BUNDLE_SKSL_PATH'];
   final bool trackWidgetCreation = Platform.environment['TRACK_WIDGET_CREATION'] == 'true';
   final bool treeShakeIcons = Platform.environment['TREE_SHAKE_ICONS'] == 'true';
   final bool verbose = Platform.environment['VERBOSE_SCRIPT_LOGGING'] == 'true';
   final bool prefixedErrors = Platform.environment['PREFIXED_ERROR_LOGGING'] == 'true';
 
+  if (projectDirectory == null) {
+    stderr.write('PROJECT_DIR environment variable must be set to the location of Flutter project to be built.');
+    exit(1);
+  }
+  if (flutterRoot == null || flutterRoot.isEmpty) {
+    stderr.write('FLUTTER_ROOT environment variable must be set to the location of the Flutter SDK.');
+    exit(1);
+  }
+
   Directory.current = projectDirectory;
 
   if (localEngine != null && !localEngine.contains(buildMode)) {
diff --git a/packages/flutter_tools/test/integration.shard/tool_backend_test.dart b/packages/flutter_tools/test/integration.shard/tool_backend_test.dart
new file mode 100644
index 0000000000..d91e6e5092
--- /dev/null
+++ b/packages/flutter_tools/test/integration.shard/tool_backend_test.dart
@@ -0,0 +1,60 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// @dart = 2.8
+
+import 'package:flutter_tools/src/base/io.dart';
+
+import '../src/common.dart';
+import 'test_utils.dart';
+
+final String toolBackend = fileSystem.path.join(getFlutterRoot(), 'packages', 'flutter_tools', 'bin', 'tool_backend.dart');
+final String examplePath = fileSystem.path.join(getFlutterRoot(), 'examples', 'hello_world');
+final String dart = fileSystem.path.join(getFlutterRoot(), 'bin', platform.isWindows ? 'dart.bat' : 'dart');
+
+void main() {
+  testWithoutContext('tool_backend.dart exits if PROJECT_DIR is not set', () async {
+    final ProcessResult result = await processManager.run(<String>[
+      dart,
+      toolBackend,
+      'linux-x64',
+      'debug',
+    ]);
+
+    expect(result.exitCode, 1);
+    expect(result.stderr, 'PROJECT_DIR environment variable must be set to the location of Flutter project to be built.');
+  });
+
+  testWithoutContext('tool_backend.dart exits if FLUTTER_ROOT is not set', () async {
+    // Removing parent environment means that batch script cannot be run.
+    final String dart = fileSystem.path.join(getFlutterRoot(), 'bin', 'cache', 'dart-sdk', 'bin', platform.isWindows ? 'dart.exe' : 'dart');
+
+    final ProcessResult result = await processManager.run(<String>[
+      dart,
+      toolBackend,
+      'linux-x64',
+      'debug',
+    ], environment: <String, String>{
+      'PROJECT_DIR': examplePath,
+    }, includeParentEnvironment: false); // Prevent FLUTTER_ROOT set by test environment from leaking
+
+    expect(result.exitCode, 1);
+    expect(result.stderr, 'FLUTTER_ROOT environment variable must be set to the location of the Flutter SDK.');
+  });
+
+  testWithoutContext('tool_backend.dart exits if local engine does not match build mode', () async {
+    final ProcessResult result = await processManager.run(<String>[
+      dart,
+      toolBackend,
+      'linux-x64',
+      'debug',
+    ], environment: <String, String>{
+      'PROJECT_DIR': examplePath,
+      'LOCAL_ENGINE': 'release_foo_bar', // Does not contain "debug",
+    });
+
+    expect(result.exitCode, 1);
+    expect(result.stderr, contains('ERROR: Requested build with Flutter local engine at \'release_foo_bar\''));
+  });
+}