Cannot build Dart as a shared library on Windows

[ds84182]

Locally I patch:

runtime/BUILD.gn:

• include library dependencies (for example ole32.lib) in the dart_shared_lib config
• include bin:dart_io_api (since Dart without dart:io is unfortunately "awful" to deal with, also requires exporting symbols in dart_io_api.h)

While this patch is mostly sustainable, Dart's tests that directly link to dart.lib on Windows need to also link an appropriate DLL library. e.g. libdart_jit.dll.lib.

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The reason why I build Dart as a shared library is to reduce iteration time & enable usage outside of GN. It works well, other than these build-related issues. Of course this use-case is somewhat atypical, but isn't much of a burden to support (I think shared objects build fine on Linux).