Add emsdk Manifest-only build to VMR

We want to be sure that VMR builds include our baseline manifests - for WASM support, that means brining in the emsdk repo and building the manifest nupkgs from there (but not the "full" build, which right now isn't truly from source anyway)

src/SourceBuild/content/repo-projects/emsdk.proj

@@ -0,0 +1,31 @@
+<Project>
+  <Import Project="Sdk.props" Sdk="Microsoft.NET.Sdk" />
+
+  <PropertyGroup>
+    <OverrideTargetRid>$(TargetRid)</OverrideTargetRid>
+    <OverrideTargetRid Condition="'$(TargetOS)' == 'OSX'">osx-$(Platform)</OverrideTargetRid>
+    <OverrideTargetRid Condition="'$(TargetOS)' == 'FreeBSD'">freebsd-$(Platform)</OverrideTargetRid>
+    <OverrideTargetRid Condition="'$(TargetOS)' == 'Windows_NT'">win-$(Platform)</OverrideTargetRid>
+
+    <_platformIndex>$(OverrideTargetRid.LastIndexOf('-'))</_platformIndex>
+    <TargetOS>$(NETCoreSdkRuntimeIdentifier.Substring(0, $(_platformIndex)))</TargetOS>
+    <TargetArch>$(NETCoreSdkPortableRuntimeIdentifier.Substring($(_platformIndex)))</TargetArch>
+
+    <BuildCommandArgs>$(StandardSourceBuildArgs)</BuildCommandArgs>
+    <BuildCommandArgs>$(BuildCommandArgs) /p:PackageRid=$(OverrideTargetRid)</BuildCommandArgs>
+    <BuildCommandArgs>$(BuildCommandArgs) /p:AssetManifestOS=$(TargetOS)</BuildCommandArgs>
+    <BuildCommandArgs>$(BuildCommandArgs) /p:PlatformName=$(TargetArch)</BuildCommandArgs>
+    <BuildCommandArgs>$(BuildCommandArgs) /p:ForceBuildManifestOnly=true</BuildCommandArgs>
+    <BuildCommand>$(StandardSourceBuildCommand) $(BuildCommandArgs)</BuildCommand>
+
+    <GlobalJsonFile>$(ProjectDirectory)global.json</GlobalJsonFile>
+    <NuGetConfigFile>$(ProjectDirectory)NuGet.config</NuGetConfigFile>
+    <OutputPlacementRepoApiImplemented>false</OutputPlacementRepoApiImplemented>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <UseSourceBuiltSdkOverride Include="@(ArcadeSdkOverride)" />
+  </ItemGroup>
+
+  <Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk" />
+</Project>

src/SourceBuild/content/repo-projects/known-good.proj

@@ -34,6 +34,7 @@
         <!-- Tier 1 -->
         <RepositoryReference Include="command-line-api" />
         <RepositoryReference Include="diagnostics" />
+        <RepositoryReference Include="emsdk" />
         <RepositoryReference Include="razor" />
         <RepositoryReference Include="xliff-tasks" />
         <RepositoryReference Include="runtime" />

src/VirtualMonoRepo/source-mappings.json

@@ -75,6 +75,10 @@
             "name": "diagnostics",
             "defaultRemote": "https://github.com/dotnet/diagnostics"
         },
+        {
+            "name": "emsdk",
+            "defaultRemote": "https://github.com/dotnet/emsdk"
+        },
         {
             "name": "format",
             "defaultRemote": "https://github.com/dotnet/format"