clean up `--dart-define-from-file` option tests (#135980)

Fixes https://github.com/flutter/flutter/issues/134279.

Changes:
* Moves all tests of `--dart-define-from-file` behavior from `build_bundle_test.dart` and `assemble_test.dart` to `flutter_command_test.dart`.
* Deletes a duplicate test of malformed JSON detection behavior.
* Renames the `useDartDefineFromFileOption` method of `FlutterCommand` to `_usesDartDefineFromFileOption`. This 1) makes the name more consistent with the other `uses*Option` methods and 2) hides the method since it is not used outside of the file.
* Renames several tests to better articulate what is under test and what the expected result is.
* Adds a test for the case where a `.env` file with a malformed line is provided to `--dart-define-from-file`.

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

@@ -656,10 +656,10 @@ abstract class FlutterCommand extends Command<void> {
       valueHelp: 'foo=bar',
       splitCommas: false,
     );
-    useDartDefineFromFileOption();
+    _usesDartDefineFromFileOption();
   }
 
-  void useDartDefineFromFileOption() {
+  void _usesDartDefineFromFileOption() {
     argParser.addMultiOption(
       FlutterOptions.kDartDefineFromFileOption,
       help:
@@ -1441,9 +1441,10 @@ abstract class FlutterCommand extends Command<void> {
             dartDefineConfigJsonMap[key] = value;
           });
         } on FormatException catch (err) {
-          throwToolExit('Json config define file "--${FlutterOptions
-              .kDartDefineFromFileOption}=$path" format err, '
-              'please fix first! format err:\n$err');
+          throwToolExit('Unable to parse the file at path "$path" due to a formatting error. '
+            'Ensure that the file contains valid JSON.\n'
+            'Error details: $err'
+          );
         }
       }
     }

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

@@ -290,49 +290,4 @@ void main() {
       ],
     });
   });
-
-  testUsingContext('test --dart-define-from-file option with err file format', () {
-    globals.fs.directory('config').createSync();
-    final CommandRunner<void> commandRunner = createTestCommandRunner(AssembleCommand(
-      buildSystem: TestBuildSystem.all(BuildResult(success: true)),
-    ));
-
-    expect(commandRunner.run(<String>['assemble',
-      '-o Output',
-      'debug_macos_bundle_flutter_assets',
-      '--dart-define=k=v',
-      '--dart-define-from-file=config']),
-        throwsToolExit(message: 'Did not find the file passed to "--dart-define-from-file". Path: config'));
-  }, overrides: <Type, Generator>{
-    Cache: () => Cache.test(processManager: FakeProcessManager.any()),
-    FileSystem: () => MemoryFileSystem.test(),
-    ProcessManager: () => FakeProcessManager.any(),
-  });
-
-  testUsingContext('test --dart-define-from-file option with err json format', () async {
-    await globals.fs.file('config.json').writeAsString(
-        '''
-          {
-            "kInt": 1,
-            "kDouble": 1.1,
-            "name": "err json format,
-            "title": "this is title from config json file"
-          }
-        '''
-    );
-    final CommandRunner<void> commandRunner = createTestCommandRunner(AssembleCommand(
-      buildSystem: TestBuildSystem.all(BuildResult(success: true)),
-    ));
-
-    expect(commandRunner.run(<String>['assemble',
-      '-o Output',
-      'debug_macos_bundle_flutter_assets',
-      '--dart-define=k=v',
-      '--dart-define-from-file=config.json']),
-        throwsToolExit(message: 'Json config define file "--dart-define-from-file=config.json" format err, please fix first! format err:'));
-  }, overrides: <Type, Generator>{
-    Cache: () => Cache.test(processManager: FakeProcessManager.any()),
-    FileSystem: () => MemoryFileSystem.test(),
-    ProcessManager: () => FakeProcessManager.any(),
-  });
 }