The original code in DecompressionHandler.cs is the MIT-licensed code from https://github.com/dotnet/corefx. This file is not built within this repository.
Instead, the derived works in the Apache-licensed StreamReplacingHttpContent.cs and GzipDeflateHandler.cs are used. (This separation allows the reuse of stream replacement in other scenarios.) These source files are present in this directory to indicate that they are derived from DecompressionHandler. These derived works are included in the Google.Apis.Core assembly when built.