Bump http to 738a55b20e391c5a526b86bf4b02af6b7745b494
Bump http to 738a55b20e391c5a526b86bf4b02af6b7745b494

Changes:
```
> git log --format="%C(auto) %h %s" d6a4bf1..738a55b
 https://dart.googlesource.com/http.git/+/738a55b Increment cronet_http version (#803)
 https://dart.googlesource.com/http.git/+/eb4d457 Fix a bug where cronet_http did not set content-length correctly (#800)

```

Diff: https://dart.googlesource.com/http.git/+/d6a4bf1e8f9d6e8f372958387cea32b0a6a62cfa~..738a55b20e391c5a526b86bf4b02af6b7745b494/
Change-Id: Ic2713b83e140a3a3a25c7565baf27e063c32910e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/261200
Auto-Submit: Devon Carew <e996497f4f2225f0ee7a9f4d1696ee710b488b66@google.com>
Reviewed-by: Alexander Aprelev <dfb74c35a357c2059971a4bf7fe26b6a7c18085d@google.com>
Commit-Queue: Devon Carew <e996497f4f2225f0ee7a9f4d1696ee710b488b66@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk