Add scripts that prepares our windows VM image

ci/docker/windows_build/README.md

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+This directory should include scripts to build the docker container image used
+for building flutter/engine in our CI system (currently [Cirrus](cirrus-ci.org))
+using Windows.
+
+So far we're still waiting GKE to have Kubernetes 1.14 and Windows containers.
+
+Before that, we use GCE Windows VMs for our CI tests. This directory includes
+scripts for preparing that VM.

ci/docker/windows_build/prepare_windows_server_2016_core.ps1

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+# The following powershell script prepares the VM image
+# flutter-engine-windows-server-2016-core used by Flutter engine's
+# build_windows presubmit tests (see .cirrus.yml).
+#
+# The exact step of generating the VM image is:
+# 1. Create a "Windows Server 2016 Datacenter Core" GCE instance with 50GB disk
+# 2. RDP into that GCE instance to run this script with powershell
+#    (e.g., `powershell -File prepare_windows_server_2016_core.ps1`)
+# 3. Shutdown the instance and take an image of that instance
+#
+# Note that ` is the escape character in powershell.
+
+# Install visual studio 2017
+curl https://aka.ms/vs/15/release/vs_community.exe -o vs_community.exe
+./vs_community.exe --passive --wait `
+    --add Microsoft.VisualStudio.Workload.NativeCrossPlat `
+    --add Microsoft.VisualStudio.Workload.NativeDesktop --includeRecommended
+
+# Install debugger tools
+curl https://download.microsoft.com/download/5/C/3/5C3770A3-12B4-4DB4-BAE7-99C624EB32AD/windowssdk/winsdksetup.exe `
+    -o winsdksetup.exe
+./winsdksetup.exe /features OptionId.WindowsDesktopDebuggers /q
+
+# Download depot tools
+curl https://storage.googleapis.com/chrome-infra/depot_tools.zip `
+    -o depot_tools.zip
+Expand-Archive -LiteralPath depot_tools.zip -DestinationPath c:/depot_tools
+
+# Download git. Although depot_tools has its own git.bat, this is needed for
+# Flutter engine's gn to run correctly.
+#
+# Somehow, curl can't get the MinGit from github. Fortunately, taobo works.
+curl https://npm.taobao.org/mirrors/git-for-windows/v2.21.0.windows.1/MinGit-2.21.0-64-bit.zip `
+    -o MinGit.zip
+Expand-Archive -LiteralPath MinGit.zip -DestinationPath c:/MinGit
+
+# Restarting the terminal (or even the whole VM) is required to let following
+# environment variables to take effect.
+setx path "c:/depot_tools/;c:/MinGit/cmd;${env:path}"
+setx DEPOT_TOOLS_WIN_TOOLCHAIN 0
+setx GYP_MSVS_OVERRIDE_PATH "c:/Program Files (x86)/Microsoft Visual Studio/2017/Community"
+
+mkdir c:/flutter/engine
+curl https://raw.githubusercontent.com/flutter/engine/master/ci/docker/build/engine_gclient `
+    -o c:/flutter/engine/.gclient
+
+# Once all the environment variables above are loaded correctly, one may test
+# build the engine by:
+#   cd c:/flutter/engine
+#   gclient sync
+#   cd src
+#   python flutter/tools/gn
+#   ninja -C out/host_debug