Refact. Build flutter web (#7472)

* Refact. Build flutter web

Signed-off-by: fufesou <shuanglongchen@yeah.net>

* Refact. Flutter web, wrap Platform.xx

Signed-off-by: fufesou <shuanglongchen@yeah.net>

---------

Signed-off-by: fufesou <shuanglongchen@yeah.net>

flutter/lib/web/common.dart

@@ -0,0 +1,9 @@
+
+final isAndroid_ = false;
+final isIOS_ = false;
+final isWindows_ = false;
+final isMacOS_ = false;
+final isLinux_ = false;
+final isWeb_ = true;
+
+final isDesktop_ = false;

flutter/lib/web/plugin/handlers.dart

@@ -0,0 +1,14 @@
+abstract class NativeHandler {
+  bool onEvent(Map<String, dynamic> evt);
+}
+
+class NativeUiHandler extends NativeHandler {
+  NativeUiHandler._();
+
+  static NativeUiHandler instance = NativeUiHandler._();
+
+  @override
+  bool onEvent(Map<String, dynamic> evt) {
+      throw UnimplementedError();
+  }
+}

flutter/lib/web/texture_rgba_renderer.dart

@@ -0,0 +1,20 @@
+import 'dart:typed_data';
+
+class TextureRgbaRenderer {
+  Future<int> createTexture(int key) {
+    throw UnimplementedError();
+  }
+
+  Future<bool> closeTexture(int key) {
+    throw UnimplementedError();
+  }
+
+  Future<bool> onRgba(
+      int key, Uint8List data, int height, int width, int strideAlign) {
+    throw UnimplementedError();
+  }
+
+  Future<int> getTexturePtr(int key) {
+    throw UnimplementedError();
+  }
+}

flutter/lib/web/win32.dart

@@ -0,0 +1,5 @@
+
+/// No use, for compilation only.
+int getWindowsTargetBuildNumber_() {
+  return 0;
+}