[flutter_tools] Don't fail copying files if the destination is not writable (#71215)

packages/flutter_tools/lib/src/base/error_handling_io.dart

@@ -289,10 +289,7 @@ class ErrorHandlingFile
     // Next check if the destination file can be written. If not, bail through
     // error handling.
     _runSync<void>(
-      ()  {
-        resultFile.createSync(recursive: true);
-        resultFile.openSync(mode: FileMode.writeOnly).closeSync();
-      },
+      () => resultFile.createSync(recursive: true),
       platform: _platform,
       failureMessage: 'Flutter failed to copy $path to $newPath due to destination location error'
     );

packages/flutter_tools/test/general.shard/base/error_handling_io_test.dart

@@ -817,12 +817,12 @@ void main() {
       expect(() => fileSystem.file('source').copySync('dest'), throwsToolExit());
     });
 
-    testWithoutContext('copySync handles error if openSync on destination file fails', () {
+    testWithoutContext('copySync handles error if createSync on destination file fails', () {
       final MockFile source = MockFile();
       final MockFile dest = MockFile();
       when(source.openSync(mode: anyNamed('mode')))
         .thenReturn(MockRandomAccessFile());
-      when(dest.openSync(mode: anyNamed('mode')))
+      when(dest.createSync(recursive: anyNamed('recursive')))
         .thenThrow(const FileSystemException('', '', OSError('', eaccess)));
       when(mockFileSystem.file('source')).thenReturn(source);
       when(mockFileSystem.file('dest')).thenReturn(dest);
@@ -830,6 +830,24 @@ void main() {
       expect(() => fileSystem.file('source').copySync('dest'), throwsToolExit());
     });
 
+    // dart:io is able to clobber read-only files.
+    testWithoutContext('copySync will copySync even if the destination is not writable', () {
+      final MockFile source = MockFile();
+      final MockFile dest = MockFile();
+
+      when(source.copySync(any)).thenReturn(dest);
+      when(mockFileSystem.file('source')).thenReturn(source);
+      when(source.openSync(mode: anyNamed('mode')))
+        .thenReturn(MockRandomAccessFile());
+      when(mockFileSystem.file('dest')).thenReturn(dest);
+      when(dest.openSync(mode: FileMode.writeOnly))
+        .thenThrow(const FileSystemException('', '', OSError('', eaccess)));
+
+      fileSystem.file('source').copySync('dest');
+
+      verify(source.copySync('dest')).called(1);
+    });
+
     testWithoutContext('copySync will copySync if there are no exceptions', () {
       final MockFile source = MockFile();
       final MockFile dest = MockFile();