Fixes to make Google.Protobuf.dll compatible with .NET 3.5

[JohnHBrock]

I changed the DOTNET35 conditional compilation symbols to NET35 because the NET35 seems to be defined in .NET Core projects by default, whereas DOTNET35 requires a custom definition.

I also explicitly added net35 to the project.json frameworks for Google.Protobuf and Google.Protobuf.Test, and updated the csharp README with detailed testing information.

Not sure if you guys are doing continuous integration with the C# stuff, but if so, you may need to make modifications after merging this to account for dotnet-test-nunit's lack of .NET 3.5 support. EDIT: Looks like I was able to make the appropriate modifications needed to keep Travis and AppVeyor happy.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[bazel-io]

Can one of the admins verify this patch?

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[JohnHBrock]

I signed it!

[googlebot]

CLAs look good, thanks!

[JohnHBrock]

@jskeet I was told by @detlevschwabe that I should ask you to review this PR.

[jskeet]

We have deliberately not been including .NET 3.5 in the project.json file - it's not a fully supported platform for us, and in particular we know there are ways in which it fails on Unity, e.g. due to the use of expression trees.

Basically, I'm fine with making changes so that the code does compile with .NET 3.5 again, but I don't want that as part of the regular build.

In terms of code that actually needs changing, I'm aware of Stream.CopyTo, and I'm happy to accept a PR that just has that. The test that uses GetTypeInfo() may be fixable without conditionally removing that part of the test itself, but I guess it's not a significant removal.

[JohnHBrock]

Sure, I'll add a section to the README explaining the status of .NET 3.5 support and remove my modifications to the project.json and appveyor files.

FWIW, here's my argument for officially supporting .NET 3.5:

Microsoft has ended .NET 4.5.1 support, but they haven't ended .NET 3.5 SP1 support: .NET 3.5 SP1 support is tied to the various Windows OS support lifecycles, including Windows 10; some of these OS support lifecycles last through at least 2020.

.NET 3.5 is also the highest version that can still run on Windows XP. Standard Windows XP isn't supported by Microsoft anymore, but embedded versions -- especially for point-of-sale systems like cash registers -- I believe are still officially supported by Microsoft. It's hard to find firm numbers, but there are likely millions of point-of-sale systems out there running Windows XP (this is actually why I'm interested in protobuf support for .NET 3.5).

[JohnHBrock]

I removed net35 from the project.json files and rewrote the README to include information on the status of .NET 3.5 support.

[jskeet]

Will merge when I've done a little coordination internally around the change from DOTNET35 to NET35.

[jskeet]

Have confirmation - will merge when Travis has gone green.