Work around to fix appveyor build (#11295)

* work around for appveyor connectivity issues

Unfortuantelly, this slows down our build :(

* review feedback

bin/internal/update_dart_sdk.ps1

@@ -40,8 +40,13 @@ if (Test-Path $dartSdkPath) {
 }
 New-Item $dartSdkPath -force -type directory | Out-Null
 $dartSdkZip = "$cachePath\dart-sdk.zip"
-Import-Module BitsTransfer
-Start-BitsTransfer -Source $dartSdkUrl -Destination $dartSdkZip
+# TODO(goderbauer): remove (slow and backwards-incompatible) appveyor work around
+if (Test-Path Env:\APPVEYOR) {
+    curl $dartSdkUrl -OutFile $dartSdkZip
+} else {
+    Import-Module BitsTransfer
+    Start-BitsTransfer -Source $dartSdkUrl -Destination $dartSdkZip
+}
 
 Write-Host "Unzipping Dart SDK..."
 If (Get-Command 7z -errorAction SilentlyContinue) {