[Gallery] Fix directory structure (#312)

gallery/lib/constants.dart

@@ -0,0 +1,23 @@
+// Copyright 2019 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Only put constants shared between files here.
+
+// height of the 'Gallery' header
+const double galleryHeaderHeight = 64;
+
+// The font size delta for display1 font.
+const double desktopDisplay1FontDelta = 16;
+
+// The width of the settingsDesktop.
+const double desktopSettingsWidth = 520;
+
+// Sentinel value for the system text scale factor option.
+const double systemTextScaleFactorOption = -1;
+
+// The splash page animation duration.
+const splashPageAnimationDurationInMilliseconds = 300;
+
+// The desktop top padding for a page's first header (e.g. Gallery, Settings)
+const firstHeaderDesktopTopPadding = 5.0;