Allow empty dart defines in flutter assemble (flutter#177198)

When building
https://github.com/flutter/samples/tree/main/add_to_app/multiple_flutters,
the `[CP-User] Run Flutter Build multiple_flutters_module Script` step
failed with:
```
Showing Recent Messages
[   +3 ms] Improperly formatted define flag: 
[        ] 
           #0      throwToolExit (package:flutter_tools/src/base/common.dart:34:3)
           #1      AssembleCommand._parseDefines (package:flutter_tools/src/commands/assemble.dart:279:9)
```

the command was:
```
 /Users/<User>/flutter/bin/flutter --verbose assemble --no-version-check --output=/Users/<User>/Library/Developer/Xcode/DerivedData/MultipleFluttersIos-<Hash>/Build/Products/Debug-iphonesimulator/ -dTargetPlatform=ios -dTargetFile=lib/main.dart -dBuildMode=debug -dConfiguration=Debug -dIosArchs=arm64 -dSdkRoot=/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator26.0.sdk -dSplitDebugInfo= -dTreeShakeIcons=false -dTrackWidgetCreation=true -dDartObfuscation=false -dAction=build -dFrontendServerStarterPath= --ExtraGenSnapshotOptions= --DartDefines= --ExtraFrontEndOptions= -dSrcRoot=/Users/<User>/flutter_debug/samples/add_to_app/multiple_flutters/multiple_flutters_ios -dTargetDeviceOSVersion=26.0 -dCodesignIdentity=- debug_ios_bundle_flutter_assets
```

the empty `--DartDefines=` comes from
https://github.com/flutter/flutter/blob/68cddc9bf5c449d5a55fe48ebb9b446ef76ae6b0/packages/flutter_tools/bin/xcode_backend.dart#L660

This PR makes `_parseDefines` ignore these empty dart defines.

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Author: LongCatIsLooong

packages/flutter_tools/lib/src/commands/assemble.dart

@@ -249,7 +249,7 @@ class AssembleCommand extends FlutterCommand {
       output = globals.fs.path.join(_flutterProject.directory.path, output);
     }
     final Artifacts artifacts = globals.artifacts!;
-    final result = Environment(
+    return Environment(
       outputDir: globals.fs.directory(output),
       buildDir: _flutterProject.directory
           .childDirectory('.dart_tool')
@@ -269,12 +269,14 @@ class AssembleCommand extends FlutterCommand {
       engineVersion: artifacts.usesLocalArtifacts ? null : globals.flutterVersion.engineRevision,
       generateDartPluginRegistry: true,
     );
-    return result;
   }
 
   Map<String, String> _parseDefines(List<String> values) {
     final results = <String, String>{};
     for (final chunk in values) {
+      if (chunk.isEmpty) {
+        continue;
+      }
       final int indexEquals = chunk.indexOf('=');
       if (indexEquals == -1) {
         throwToolExit('Improperly formatted define flag: $chunk');

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

@@ -1603,7 +1603,10 @@ abstract class FlutterCommand extends Command<void> {
     });
 
     if (argParser.options.containsKey(FlutterOptions.kDartDefinesOption)) {
-      dartDefines.addAll(stringsArg(FlutterOptions.kDartDefinesOption));
+      final Iterable<String> defines = stringsArg(
+        FlutterOptions.kDartDefinesOption,
+      ).where((string) => string.isNotEmpty);
+      dartDefines.addAll(defines);
     }
 
     return dartDefines;

packages/flutter_tools/test/commands.shard/hermetic/assemble_test.dart

@@ -85,6 +85,35 @@ void main() {
     },
   );
 
+  testUsingContext(
+    'flutter assemble can parse empty defines',
+    () async {
+      final CommandRunner<void> commandRunner = createTestCommandRunner(
+        AssembleCommand(
+          buildSystem: TestBuildSystem.all(BuildResult(success: true), (
+            Target target,
+            Environment environment,
+          ) {
+            expect(environment.defines, const {'DeferredComponents': 'false'});
+          }),
+        ),
+      );
+      await commandRunner.run(<String>[
+        'assemble',
+        '-o Output',
+        '--DartDefines=',
+        'debug_macos_bundle_flutter_assets',
+      ]);
+
+      expect(testLogger.traceText, contains('build succeeded.'));
+    },
+    overrides: <Type, Generator>{
+      Cache: () => Cache.test(processManager: FakeProcessManager.any()),
+      FileSystem: () => MemoryFileSystem.test(),
+      ProcessManager: () => FakeProcessManager.any(),
+    },
+  );
+
   testUsingContext(
     'flutter assemble can parse inputs',
     () async {