Check powershell version. (#27596)

bin/internal/update_dart_sdk.ps1

@@ -23,6 +23,15 @@ $engineVersion = (Get-Content "$flutterRoot\bin\internal\engine.version")
 
 $oldDartSdkPrefix = "dart-sdk.old"
 
+# Make sure that PowerShell has expected version.
+$psMajorVersionRequired = 5
+$psMajorVersionLocal = $PSVersionTable.PSVersion.Major
+if ($psMajorVersionLocal -lt $psMajorVersionRequired) {
+    Write-Host "Flutter requires PowerShell $psMajorVersionRequired.0 or newer."
+    Write-Host "See https://flutter.io/docs/get-started/install/windows for more."
+    return
+}
+
 if ((Test-Path $engineStamp) -and ($engineVersion -eq (Get-Content $engineStamp))) {
     return
 }