Fix incorrectly checking for invalid environment variables in the tool (#164101)

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This PR removes a check that looks for the use of Flutter version
related Dart define keys in the environment variables (not Dart defines)
of the user of the Flutter tool. We don't need to check the environment
variables, we only need to check the user defined Dart defines.

Here's how it currently works:

1. User builds a Flutter app via tool
2. tool checks environment variables for Flutter version related Dart
define keys and throws if it finds and (_and_)
3. tool checks list of user defined Dart defines for Flutter version
related Dart define keys and throws if it finds any

This PR removes the check in step 2, since step 3 is what we actually
care about.

The `FLUTTER_GIT_URL` environment variable is set on forks and the Dart
define is used to make that information available to applications at
runtime. However, environment variables don't propagate to be Dart
defines, thus the above mentioned check is not needed.

*List which issues are fixed by this PR. You must list at least one
issue. An issue is not required if the PR fixes something trivial like a
typo.*

Fixes #164093

*If you had to change anything in the [flutter/tests] repo, include a
link to the migration guide as per the [breaking change policy].*

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

@@ -1480,12 +1480,6 @@ abstract class FlutterCommand extends Command<void> {
   // This adds the Dart defines used to access various Flutter version information at runtime.
   void _addFlutterVersionToDartDefines(FlutterVersion version, List<String> dartDefines) {
     for (final String dartDefine in flutterVersionDartDefines) {
-      if (globals.platform.environment[dartDefine] != null) {
-        throwToolExit(
-          '$dartDefine is used by the framework and cannot be set in the environment. '
-          'Use FlutterVersion to access it in Flutter code',
-        );
-      }
       if (dartDefines.any((String define) => define.startsWith(dartDefine))) {
         throwToolExit(
           '$dartDefine is used by the framework and cannot be '

packages/flutter_tools/test/general.shard/runner/flutter_command_test.dart

@@ -1272,39 +1272,6 @@ flutter:
 
     group('Flutter version', () {
       for (final String dartDefine in FlutterCommand.flutterVersionDartDefines) {
-        testUsingContext(
-          "tool exits when $dartDefine is already set in user's environment",
-          () async {
-            final CommandRunner<void> runner = createTestCommandRunner(
-              _TestRunCommandThatOnlyValidates(),
-            );
-
-            await expectLater(
-              runner.run(<String>['run', '--no-pub', '--no-hot']),
-              throwsToolExit(
-                message:
-                    '$dartDefine is used by the framework and cannot be set in the environment. '
-                    'Use FlutterVersion to access it in Flutter code',
-              ),
-            );
-          },
-          overrides: <Type, Generator>{
-            DeviceManager:
-                () => FakeDeviceManager()..attachedDevices = <Device>[FakeDevice('name', 'id')],
-            Platform:
-                () => FakePlatform(environment: <String, String>{dartDefine: 'I was already set'}),
-            Cache: () => Cache.test(processManager: FakeProcessManager.any()),
-            FileSystem: () {
-              return MemoryFileSystem.test()
-                ..file('lib/main.dart').createSync(recursive: true)
-                ..file('pubspec.yaml').createSync()
-                ..file('.packages').createSync();
-            },
-            ProcessManager: () => FakeProcessManager.any(),
-            FlutterVersion: () => FakeFlutterVersion(),
-          },
-        );
-
         testUsingContext(
           'tool exits when $dartDefine is set in --dart-define or --dart-define-from-file',
           () async {
@@ -1361,6 +1328,38 @@ flutter:
         );
       }
 
+      // Regression test for https://github.com/flutter/flutter/issues/164093
+      testUsingContext(
+        'tool does not throw when FLUTTER_GIT_URL exists in environment variables',
+        () async {
+          final CommandRunner<void> runner = createTestCommandRunner(
+            _TestRunCommandThatOnlyValidates(),
+          );
+
+          await expectReturnsNormallyLater(runner.run(<String>['run', '--no-pub', '--no-hot']));
+        },
+        overrides: <Type, Generator>{
+          DeviceManager:
+              () => FakeDeviceManager()..attachedDevices = <Device>[FakeDevice('name', 'id')],
+          Platform:
+              () =>
+                  FakePlatform()
+                    ..environment = <String, String>{
+                      'FLUTTER_GIT_URL': 'git@example.org:fork_of/flutter.git',
+                    },
+          Cache: () => Cache.test(processManager: FakeProcessManager.any()),
+          FileSystem: () {
+            final MemoryFileSystem fileSystem = MemoryFileSystem.test();
+            fileSystem.file('lib/main.dart').createSync(recursive: true);
+            fileSystem.file('pubspec.yaml').createSync();
+            fileSystem.file('.packages').createSync();
+            return fileSystem;
+          },
+          ProcessManager: () => FakeProcessManager.any(),
+          FlutterVersion: () => FakeFlutterVersion(),
+        },
+      );
+
       testUsingContext(
         'FLUTTER_VERSION is set in dartDefines',
         () async {