[build] r8 now builds on both Windows & MacOS

Context: http://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools_tutorial.html

Overview of changes:
- Fixed tab characters, formatting, spacing
- To get `depot_tools`, we now just use the download on all
  platforms--not using a submodule anymore
- Use the `UnzipDirectoryChildren` task from `xa-prep-tasks`, so
  `r8.csproj` doesn't depend on `Xamarin.Android.Build.Tasks`
- `_BootstrapDepotTools` runs the `gclient` command, which *on first
  run* downloads all dependencies required for `depot_tools`, such as
  gradle. This was a hard requirement for Windows especially, since it
  downloads python.
- Use the `SetEnvironmentVariable` task to configure `PATH`.
  `depot_tools` has an odd requirement of being in `PATH`, but the
  `EnvironmentVariables` property on the `<Exec />` task was not
  sufficient. On Windows, the way `external\r8\tools\gradle.py`
  invokes `.bat` files in `depot_tools`--I could not get things to
  work unless doing it this way.
- Various targets for `depot_tools` use Inputs/Outputs so they aren't
  running every time.
- Removed `depot_tools.tpnitems` to `src`, since it is no longer a
  submodule and we don't redistribute it. Is that correct?

.gitmodules

@@ -41,9 +41,6 @@
 	path = external/xamarin-android-tools
 	url = https://github.com/xamarin/xamarin-android-tools
 	branch = master
-[submodule "external/depot_tools"]
-	path = external/depot_tools
-	url = https://chromium.googlesource.com/chromium/tools/depot_tools.git
 [submodule "external/r8"]
 	path = external/r8
 	url = https://r8.googlesource.com/r8

Xamarin.Android.sln

@@ -124,6 +124,7 @@ EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Xamarin.Android.Tools.AndroidSdk-Tests", "external\xamarin-android-tools\src\Xamarin.Android.Tools.AndroidSdk\Tests\Xamarin.Android.Tools.AndroidSdk-Tests.csproj", "{1E5501E8-49C1-4659-838D-CC9720C5208F}"
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "r8", "src\r8\r8.csproj", "{1BAFA0CC-0377-46CE-AB7B-7BB2E7B62F63}"
+EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "proprietary", "build-tools\proprietary\proprietary.csproj", "{D93CAC27-3893-42A3-99F1-2BCA72E186F4}"
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "download-bundle", "build-tools\download-bundle\download-bundle.csproj", "{1DA0CB12-5508-4E83-A242-0C8D6D99A49B}"

src/r8/r8.targets

@@ -1,76 +1,116 @@
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 
-  <UsingTask TaskName="Xamarin.Android.Tasks.Unzip" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
-  <UsingTask AssemblyFile="$(MSBuildThisFileDirectory)..\..\bin\Build$(Configuration)\xa-prep-tasks.dll"  TaskName="Xamarin.Android.BuildTools.PrepTasks.DownloadUri" />
+  <UsingTask AssemblyFile="..\..\bin\Build$(Configuration)\xa-prep-tasks.dll" TaskName="Xamarin.Android.BuildTools.PrepTasks.DownloadUri" />
+  <UsingTask AssemblyFile="..\..\bin\Build$(Configuration)\xa-prep-tasks.dll" TaskName="Xamarin.Android.BuildTools.PrepTasks.SetEnvironmentVariable" />
+  <UsingTask AssemblyFile="..\..\bin\Build$(Configuration)\Xamarin.Android.Tools.BootstrapTasks.dll" TaskName="Xamarin.Android.Tools.BootstrapTasks.UnzipDirectoryChildren" />
 
   <!-- public targets -->
 
-  <Target Name="Build" DependsOnTargets="_SetupDepotTools;_BuildR8;_CopyR8" />
-  <Target Name="Clean" DependsOnTargets="_CleanR8;_CleanDepotTools" />
+  <PropertyGroup>
+    <BuildDependsOn>
+      _DownloadDepotTools;
+      _UnzipDepotTools;
+      _SetDepotToolsEnvironment;
+      _BootstrapDepotTools;
+      _BuildR8;
+      _CopyR8;
+    </BuildDependsOn>
+    <CleanDependsOn>
+      _SetDepotToolsEnvironment;
+      _CleanR8;
+      _CleanDepotTools;
+    </CleanDependsOn>
+  </PropertyGroup>
+  <Target Name="Build" DependsOnTargets="$(BuildDependsOn)" />
+  <Target Name="Clean" DependsOnTargets="$(CleanDependsOn)" />
 
-  <!-- depot_tools -->
+  <!-- 
+    depot_tools, a set of git extensions from Chromium
+    http://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools_tutorial.html
+  -->
 
-  <!-- Why do people split files and make build scripts unreadable? That's unacceptable. Therefore properties are here. Always consider code readability. -->
   <PropertyGroup>
-    <_WindowsDepotToolzZipDownload>..\..\bin\Build$(Configuration)\depot_tools.zip</_WindowsDepotToolzZipDownload>
+    <_DepotToolsZip>..\..\bin\Build$(Configuration)\depot_tools.zip</_DepotToolsZip>
+    <_PathToDepotTools>$([System.IO.Path]::GetFullPath('$(MSBuildThisFileDirectory)..\..\bin\Build$(Configuration)\depot_tools'))</_PathToDepotTools>
   </PropertyGroup>
 
-  <Target Name="_SetupDepotTools" DependsOnTargets="_SetupDepotToolsWindows" />
-  <Target Name="_CleanDepotTools" DependsOnTargets="_CleanDepotToolsWindows" />
-
-  <Target Name="_SetupDepotToolsWindows" Condition="$(HostOS) == 'Windows'">
+  <Target Name="_DownloadDepotTools">
     <DownloadUri
-		SourceUris="https://storage.googleapis.com/chrome-infra/depot_tools.zip"
-		DestinationFiles="$(_WindowsDepotToolzZipDownload)"
+        SourceUris="https://storage.googleapis.com/chrome-infra/depot_tools.zip"
+        DestinationFiles="$(_DepotToolsZip)"
     />
-    <Unzip
-		Sources="$(_WindowsDepotToolsZipDownload)" 
-		Destination="bin\Build$(Configuration)\depot_tools"
-	/>
   </Target>
 
-  <Target Name="_CleanDepotToolsWindows" Condition="$(HostOS) == 'Windows'">
-    <RemoveDir Directories="bin\Build$(Configuration)\depot_tools" />
+  <Target Name="_UnzipDepotTools"
+      Inputs="$(_DepotToolsZip)"
+      Outputs="$(_PathToDepotTools)\unzip.stamp">
+    <UnzipDirectoryChildren
+        HostOS="$(HostOS)"
+        NoSubdirectory="True"
+        SourceFiles="$(_DepotToolsZip)"
+        DestinationFolder="$(_PathToDepotTools)"
+    />
+    <Touch
+        Files="$(_PathToDepotTools)\unzip.stamp"
+        AlwaysCreate="True"
+    />
   </Target>
 
+  <!--
+    NOTE: depot_tools has an odd requirement of being in PATH
+    I am also getting some odd failures on Windows, if not specifying namespace for <SetEnvironmentVariable />
+  -->
 
-  <!-- r8 -->
+  <Target Name="_SetDepotToolsEnvironment">
+    <Xamarin.Android.BuildTools.PrepTasks.SetEnvironmentVariable
+        Name="PATH"
+        Value="$(_PathToDepotTools)$(PathSeparator)$(PATH)"
+    />
+  </Target>
 
-  <PropertyGroup>
-    <_PathToDepotTools Condition="'$(HostOS)' == 'Windows'">..\..\bin\Build$(Configuration)\depot_tools</_PathToDepotTools>
-    <_PathToDepotTools Condition="'$(HostOS)' != 'Windows'">..\..\external\depot_tools</_PathToDepotTools>
-    <_Sep Condition="'$(HostOS)' == 'Windows'">;</_Sep>
-    <_Sep Condition="'$(HostOS)' != 'Windows'">:</_Sep>
-    <_W Condition="'$(HostOS)' == 'Windows'">"</_W>
-    <_W Condition="'$(HostOS)' != 'Windows'"></_W>    <_EnvForDepotTools>PATH=$(_W)$(_PathToDepotTools)$(_W)$(_Sep)$(PATH)</_EnvForDepotTools>
-  </PropertyGroup>
+  <!-- This "bootstraps" depot_tools, such as downloading/installing gradle and python -->
 
-  <Target Name="_BuildR8" DependsOnTargets="_SetupDepotTools">
-   <Exec
-	Command="..\..\external\r8\tools\gradle.py d8 r8"
-	WorkingDirectory="..\..\external\r8"
-	EnvironmentVariables="$(_EnvForDepotTools)"
-   />
+  <Target Name="_BootstrapDepotTools"
+      Inputs="$(_PathToDepotTools)\unzip.stamp"
+      Outputs="$(_PathToDepotTools)\bootstrap.stamp">
+    <Exec Command="gclient --version" />
+    <Touch
+        Files="$(_PathToDepotTools)\bootstrap.stamp"
+        AlwaysCreate="True"
+    />
+  </Target>
+
+  <Target Name="_CleanDepotTools">
+    <Delete Files="$(_DepotToolsZip)" />
+    <RemoveDir Directories="$(_PathToDepotTools)" />
+  </Target>
+
+  <!-- r8 -->
+
+  <Target Name="_BuildR8">
+    <Exec
+      Command="python tools\gradle.py d8 r8"
+      WorkingDirectory="..\..\external\r8"
+    />
   </Target>
 
   <Target Name="_CopyR8">
     <Copy
-		SourceFiles="..\..\external\r8\build\libs\d8.jar"
-		DestinationFolder="$(XAInstallPrefix)\xbuild\Xamarin\Android\"
-		SkipUnchangedFiles="true"
+        SourceFiles="..\..\external\r8\build\libs\d8.jar"
+        DestinationFolder="$(XAInstallPrefix)\xbuild\Xamarin\Android\"
+        SkipUnchangedFiles="true"
     />
     <Copy
-		SourceFiles="..\..\external\r8\build\libs\r8.jar"
-		DestinationFolder="$(XAInstallPrefix)\xbuild\Xamarin\Android\"
-		SkipUnchangedFiles="true"
+        SourceFiles="..\..\external\r8\build\libs\r8.jar"
+        DestinationFolder="$(XAInstallPrefix)\xbuild\Xamarin\Android\"
+        SkipUnchangedFiles="true"
     />
   </Target>
 
   <Target Name="_CleanR8">
     <Exec
-	Command="..\..\external\r8\tools\gradle.py clean"
-	WorkingDirectory="..\..\external\r8"
-	EnvironmentVariables="$(_EnvForDepotTools)"
+      Command="python tools\gradle.py clean"
+      WorkingDirectory="..\..\external\r8"
     />
   </Target>