update

Author: Guo-Zhang

dart_language/null_safety.md

@@ -0,0 +1,13 @@
+# 空安全
+
+允许为空的方法是在类型后面加`?`，比如
+
+```dart
+int? someValue = 30;
+```
+
+强制指定值非空的方法是在后面加`!`，比如
+
+```dart
+int data = someValue!; 
+```

…ls/1_1_Flutter_SDK_and_Android_Studio.md …_tools/flutter_sdk_and_android_studio.md1_Local_Developing_Tools/1_1_Flutter_SDK_and_Android_Studio.md renamed to local_developing_tools/flutter_sdk_and_android_studio.md 1_Local_Developing_Tools/1_1_Flutter_SDK_and_Android_Studio.md renamed to local_developing_tools/flutter_sdk_and_android_studio.md

@@ -0,0 +1 @@
+# 单元测试

…ti-Platform_Support/Mobile_App_to_Web.md multi-platforms/from_mobile_to_web.md3_Multi-Platform_Support/Mobile_App_to_Web.md renamed to multi-platforms/from_mobile_to_web.md 3_Multi-Platform_Support/Mobile_App_to_Web.md renamed to multi-platforms/from_mobile_to_web.md

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+# 组件测试