null_safety rebuild script (#1369) (#1371)

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

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[Flutter Style Guide]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
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[Contributors Guide]: https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

platform_channels/ios/Runner.xcodeproj/xcshareddata/xcschemes/Runner.xcscheme

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <Scheme
-   LastUpgradeVersion = "1020"
+   LastUpgradeVersion = "1300"
    version = "1.3">
    <BuildAction
       parallelizeBuildables = "YES"
@@ -27,8 +27,6 @@
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
       shouldUseLaunchSchemeArgsEnv = "YES">
-      <Testables>
-      </Testables>
       <MacroExpansion>
          <BuildableReference
             BuildableIdentifier = "primary"
@@ -38,8 +36,8 @@
             ReferencedContainer = "container:Runner.xcodeproj">
          </BuildableReference>
       </MacroExpansion>
-      <AdditionalOptions>
-      </AdditionalOptions>
+      <Testables>
+      </Testables>
    </TestAction>
    <LaunchAction
       buildConfiguration = "Debug"
@@ -61,8 +59,6 @@
             ReferencedContainer = "container:Runner.xcodeproj">
          </BuildableReference>
       </BuildableProductRunnable>
-      <AdditionalOptions>
-      </AdditionalOptions>
    </LaunchAction>
    <ProfileAction
       buildConfiguration = "Profile"