Bloomberg feedback for 6.0

### Acknowledgement

- [x] I acknowledge that issues using this template may be closed without further explanation at the maintainer's discretion.

### Comment

We evaluated the 6.0 RC releases and 6.0 is a moderate impact release for us.

| # | Change | Affects | Release notes | Packages affected | Reported as |
| - | -------------------------------------------------------------------------- | ---------------- | ------------- | ----------------- |-------------|
| 1 | Deprecation of `node` module resolution | Module Resolution| Yes | <1% | |
| 2 | Key mapping in mapped clause cause contextual types to fail | Type Checking | No | <1% | #63225 |
| 3 | Inference produces different result | Type Checking | No | <1% | #63227 |
| 4 | JS emit removes comments | Emit | No | <1% | |
| 5 | API changes hasDefaultLib removed | Type Checking | Yes | 100% | |

## Deprecation of `node` module resolution

All our projects used the `node` moduleResolution. With the release of 6.0 this was deprecated. The resolution closest to our internal module loader behavior is `bundler`.

Most of the projects did not have issue migrating, although some did due to two main causes:

1. Some import types in declarations switched to using `import('.')` instead of `import('.\index')` which we do not support
2. Some projects used declaration files for `json` files that used the extension `json.d.ts` which worked fine in node but not with bundler (and using `--allowArbitraryExtensions` requires the extension to be `d.json.ts` which we currently do not support)

However these impacted a very small number of packages.

## JS emit removes comments

Some comments above JSX elements are removed where before they were preserved. These seem like an improvement:

```ts
function Component() {
return (
// Comment is removed in 6.0

)
}
```
[Playground Link 6.0](https://www.typescriptlang.org/play/?ts=6.0.0-dev.20260311#code/JYWwDg9gTgLgBAJQKYEMDG8BmUIjgcilQ3wFgAoTAVwDsNgIa4APACgEo4BvCuPuIjCpQmrXvwkB6SXADCuEEhrxgAZwFIQEAG5IAJnGBMAbADoADOIl8APHuDaAfDcn2nVuOwoBfIA)
[Playground Link 5.0](https://www.typescriptlang.org/play/?ts=6.0.0-dev.20260311#code/JYWwDg9gTgLgBAJQKYEMDG8BmUIjgcilQ3wFgAoTAVwDsNgIa4APACgEo4BvCuPuIjCpQmrXvwkB6SXADCuEEhrxgAZwFIQEAG5IAJnGBMAbADoADOIl8APHuDaAfDcn2nVuOwoBfIA)

## API changes hasDefaultLib removed

We had a custom implementation of skipLibChecks that only checked declarations authored in the project (usually hand authored) but not those that come from a trusted central source. This relies on setting the `hasDefaultLib` flag on the declaration source files from the trusted source and using `skipDefaultLibChecks`. This approach unfortunately no longer works with 6.0 resulting in slower build times for us.

One workaround if overriding `isSourceFileDefaultLibrary` on the `program` instance. This seems to work well in 6.0. It would be useful to have a better way to exclude files from type checking in 7.0 where we will not be able to do such monkey patching. Even with the perf improvements in 7.0 it seems wasteful to check files we know are valid.

Begin with the linked Playground examples and the reported issues #63225 and #63227, then investigate the module-resolution, JavaScript emit, and Program API areas named in the report. There is no single completion criterion: the migration effects, comment removal, inference changes, and default-library checking concerns would need separate reproduction and decisions.

Written by the indexing model from the issue text.