Add ability to cross compile (#9374)

`-CrossGen` won`t work, but this gives basic ability to build for another platform.
This also means you cannot really package for another platform.

PowerShell.Common.props

@@ -111,7 +111,7 @@
 
   <PropertyGroup>
     <DefineConstants>$(DefineConstants);CORECLR</DefineConstants>
-    <IsWindows Condition="'$(OS)' == 'Windows_NT'">true</IsWindows>
+    <IsWindows Condition="'$(IsWindows)' =='true' or ( '$(IsWindows)' == '' and '$(OS)' == 'Windows_NT')">true</IsWindows>
   </PropertyGroup>
 
   <!-- Define non-windows, all configuration properties -->

build.psm1

@@ -237,7 +237,8 @@ function Start-PSBuild {
 
         [ValidatePattern("^v\d+\.\d+\.\d+(-\w+(\.\d+)?)?$")]
         [ValidateNotNullOrEmpty()]
-        [string]$ReleaseTag
+        [string]$ReleaseTag,
+        [switch]$Detailed
     )
 
     if ($PsCmdlet.ParameterSetName -eq "Default" -and !$NoPSModuleRestore)
@@ -327,10 +328,22 @@ Fix steps:
         $Arguments += "--output", (Split-Path $Options.Output)
     }
 
+    if ($Options.Runtime -like 'win*' -or ($Options.Runtime -eq 'fxdependent' -and $Environment.IsWindows)) {
+        $Arguments += "/property:IsWindows=true"
+    }
+    else {
+        $Arguments += "/property:IsWindows=false"
+    }
+
     $Arguments += "--configuration", $Options.Configuration
     $Arguments += "--framework", $Options.Framework
 
-    if (-not $SMAOnly -and $Options.Runtime -ne 'fxdependent') {
+    if ($Detailed.IsPresent)
+    {
+        $Arguments += '--verbosity', 'd'
+    }
+
+    if (-not $SMAOnly -and $Options.Runtime -notlike 'fxdependent*') {
         # libraries should not have runtime
         $Arguments += "--runtime", $Options.Runtime
     }
@@ -653,19 +666,6 @@ function New-PSOptions {
     }
     Write-Verbose "Using configuration '$Configuration'"
 
-    $PowerShellDir = if (!$Environment.IsWindows) {
-        "powershell-unix"
-    } else {
-        "powershell-win-core"
-    }
-    $Top = [IO.Path]::Combine($PSScriptRoot, "src", $PowerShellDir)
-    Write-Verbose "Top project directory is $Top"
-
-    if (-not $Framework) {
-        $Framework = "netcoreapp2.1"
-        Write-Verbose "Using framework '$Framework'"
-    }
-
     if (-not $Runtime) {
         if ($Environment.IsLinux) {
             $Runtime = "linux-x64"
@@ -691,6 +691,20 @@ function New-PSOptions {
         }
     }
 
+    $PowerShellDir = if ($Runtime -like 'win*' -or ($Runtime -eq 'fxdependent' -and $Environment.IsWindows)) {
+        "powershell-win-core"
+    } else {
+        "powershell-unix"
+    }
+
+    $Top = [IO.Path]::Combine($PSScriptRoot, "src", $PowerShellDir)
+    Write-Verbose "Top project directory is $Top"
+
+    if (-not $Framework) {
+        $Framework = "netcoreapp2.1"
+        Write-Verbose "Using framework '$Framework'"
+    }
+
     $Executable = if ($Runtime -eq 'fxdependent') {
         "pwsh.dll"
     } elseif ($Environment.IsLinux -or $Environment.IsMacOS) {

test/powershell/engine/Help/UpdatableHelpSystem.Tests.ps1

@@ -329,11 +329,13 @@ function ValidateSaveHelp
 
     $compressedFile = GetFiles -fileType "*$extension" -path $path | ForEach-Object {Split-Path $_ -Leaf}
     $expectedCompressedFile = $testCases[$moduleName].CompressedFiles
-    $expectedCompressedFile | Should -Be $compressedFile
+    $expectedCompressedFile | Should -Not -BeNullOrEmpty -Because "Test data (expectedCompressedFile) should never be null"
+    $compressedFile | Should -Be $expectedCompressedFile -Because "Save-Help for $module should download '$expectedCompressedFile'"
 
     $helpInfoFile = GetFiles -fileType "*HelpInfo.xml" -path $path | ForEach-Object {Split-Path $_ -Leaf}
     $expectedHelpInfoFile = $testCases[$moduleName].HelpInfoFiles
-    $expectedHelpInfoFile | Should -Be $helpInfoFile
+    $expectedHelpInfoFile | Should -Not -BeNullOrEmpty -Because "Test data (expectedHelpInfoFile) should never be null"
+    $helpInfoFile | Should -Be $expectedHelpInfoFile -Because "Save-Help for $module should download '$expectedHelpInfoFile'"
 }
 
 Describe "Validate Update-Help from the Web for one PowerShell Core module." -Tags @('CI', 'RequireAdminOnWindows', 'RequireSudoOnUnix') {