Show error for '/' in Windows simple file dialog

Fixes #2448

src/vs/workbench/services/dialogs/browser/simpleFileDialog.ts

@@ -765,6 +765,9 @@ export class SimpleFileDialog {
 				// File or folder doesn't exist
 				this.filePickBox.validationMessage = nls.localize('remoteFileDialog.validateNonexistentDir', 'Please enter a path that exists.');
 				return Promise.resolve(false);
+			} else if (uri.path === '/' && (await this.isWindowsOS())) {
+				this.filePickBox.validationMessage = nls.localize('remoteFileDialog.windowsDriveLetter', 'Please start the path with a drive letter.');
+				return Promise.resolve(false);
 			} else if (stat.isDirectory && !this.allowFolderSelection) {
 				// Folder selected when folder selection not permitted
 				this.filePickBox.validationMessage = nls.localize('remoteFileDialog.validateFileOnly', 'Please select a file.');