Create GN build for Dart SDK

[whesse]

"Some people want to extend the GN build to the full Dart SDK.". It is currently working to build the runtime in the flutter context.

Here is a checklist of things that we would need to add:

• Add Wiki page "Building with GN"
• Add gn binary download to repo checkout?
• Create working standalone GN build for runtime
• Analyzer + summaries
• Snapshots of tools
• Copy files to SDK directory

[whesse]

I'll do "Add gn binary download to repo checkout".

[zanderso]

Started working on this here: https://codereview.chromium.org/2350583002/

[zanderso]

The GN build can now produce the SDK on all platforms, including cross-builds, Android, and ASAN on Linux. It's wired up to both gclient runhooks and build.py. It is used as the default if the environment variable DART_USE_GN is defined, and can otherwise be selected with the flag --gn. It also produces project files for VS and XCode on Windows and Mac, respectively.

@whesse Can we get this set up on a builder or two?

[whesse]

Wonderful! I'll set up builder/testers for this. We will need documentation about how to update this and GYP (and blaze) in simple cases like adding a source file to the build of a target. But that is mostly in VM, and you guys know already how to do it.

[mfrancis107]

________ running 'D:\VRProject\depot_tools\python276_bin\python.exe sdk/tools/generate_buildfiles.py --gn' in 'D:\VRProject\dart\dartbuild'
Command failed: D:\VRProject\dart\dartbuild\sdk/buildtools/win/gn gen --check --ide=vs out\DebugIA32 --args=is_release=false is_clang=false dart_zlib_path="//runtime/bin/zlib" use_goma=false dart_use_fallback_root_certificates=true target_cpu="x86" goma_dir="None" dart_use_tcmalloc=false dart_target_arch="ia32" is_asan=false is_product=false is_debug=true dart_runtime_mode="develop" dart_host_pub_exe="D:\VRProject\dart\dartbuild\sdk\tools\sdks\win\dart-sdk\bin\pub.bat" dart_debug=true host_cpu="x86" target_os="win"
output: Traceback (most recent call last):
  File "D:/VRProject/dart/dartbuild/sdk/build/toolchain/win/setup_toolchain.py", line 154, in <module>
    main()
  File "D:/VRProject/dart/dartbuild/sdk/build/toolchain/win/setup_toolchain.py", line 121, in main
    args = _SetupScript(cpu, win_sdk_path)
  File "D:/VRProject/dart/dartbuild/sdk/build/toolchain/win/setup_toolchain.py", line 68, in _SetupScript
    return [os.path.normpath(os.path.join(os.environ['GYP_MSVS_OVERRIDE_PATH'],
  File "D:\VRProject\depot_tools\python276_bin\lib\os.py", line 423, in __getitem__
    return self.data[key.upper()]
KeyError: 'GYP_MSVS_OVERRIDE_PATH'
ERROR at //build/toolchain/win/BUILD.gn:28:18: Script returned non-zero exit code.
toolchain_data = exec_script("setup_toolchain.py",
                 ^----------
Current dir: D:/VRProject/dart/dartbuild/sdk/out/DebugIA32/
Command: D:/VRProject/depot_tools/python276_bin/python.exe -- D:/VRProject/dart/dartbuild/sdk/build/toolchain/win/setup_toolchain.py "C:\Program Files (x86)\Microsoft Visual Studio 14.0" ../../third_party/gyp/pylib/gyp/win_tool.py "C:\Program Files (x86)\Windows Kits\10" "C:\Windows\Sysnative;C:\Windows\SysWOW64" x86
Returned 1.
See //BUILD.gn:6:1: which caused the file to be included.
group("default") {
^-----------------
Error: Command 'D:\\VRProject\\depot_tools\\python276_bin\\python.exe sdk/tools/generate_buildfiles.py --gn' returned non-zero exit status 1 in D:\VRProject\dart\dartbuild

I get the above error when attempting to use the GN build on windows.

[zanderso]

@mfrancis107 Thanks for the report. Did you follow the instructions here and set DEPOT_TOOLS_WIN_TOOLCHAIN=0. It looks like open source contributors may also need to define the environment variable GYP_MSVS_OVERRIDE_PATH to be the path to Visual Studio, e.g. C:\Program Files (x86)\Microsoft Visual Studio 14.0. I'm unable to test that at the moment, but if you're able to give it a shot, that would be helpful.

[mfrancis107]

@zanderso Manually setting the environment variable worked.

However, I think this can be resolved in the build steps. It looks like in toolchain\win\BUILD.gn a call to exec_script("setup_toolchain.py .... which uses the environment variable GYP_MSVS_OVERRIDE_PATH is called before exec_script("vs_toolchain.py".... which sets the environment variable if it is not already set.

[mit-mit]

@whesse @zanderso let's get this completed -- did you look at the env var issue mentioned above?

[zanderso]

I've created a new issue for the environment variable situation, which is not a blocker for the GN build. There may also still be a few other small issues remaining with the GN build, but we should file issues for those separately as well as they come up, so I'll go ahead and close this one.