This pull request is created by [automatic cherry pick workflow](https://github.com/flutter/flutter/wiki/Flutter-Cherrypick-Process#automatically-creates-a-cherry-pick-request)

### Issue Link:
https://github.com/flutter/flutter/issues/145653

### Changelog Description:
Fixes and issue where dart2js deferred part files were not being properly emitted by a web build.

### Impact Description:
Users who use deferred loading in dart2js will not have any of the deferred JS files copied to their build directory.

### Workaround:
Users can manually copy the part files out of the `.dart_tool/flutter_build` directory into their `build/web` folder.

### Risk:
What is the risk level of this cherry-pick?

### Test Coverage:
Are you confident that your fix is well-tested by automated tests?

### Validation Steps:
Follow the repro steps listed in https://github.com/flutter/flutter/issues/145653.
Ensure that the `build/web` folder contains the `main.dart.js_1.part.js` and `main.dart.js_1.part.js.map` files.