Fix broken Linux build (flutter/engine#5019)

After we write the pixels in the correct format,
the color type of the SkPixmap is still set to
its previous value, so the existing assertion was
failing.

engine/src/flutter/.gitignore

@@ -5,6 +5,7 @@
 .DS_Store
 .classpath
 .cproject
+.dart_tool
 .gdb_history
 .checkstyle
 .gdbinit

engine/src/flutter/lib/ui/painting/image_encoding.cc

@@ -79,7 +79,6 @@ sk_sp<SkData> GetImageBytesAsRGBA(sk_sp<SkImage> image) {
       FXL_LOG(ERROR) << "Pixel address is not available.";
       return nullptr;
     }
-    ASSERT(pixmap.colorType() == kRGBA_8888_SkColorType);
 
     const size_t pixmap_size = pixmap.computeByteSize();
     return SkData::MakeWithCopy(pixmap.addr32(), pixmap_size);

engine/src/flutter/testing/dart/.gitignore

@@ -1,3 +1,2 @@
-.dart_tool
 .packages
 pubspec.lock