Better handling of user provided paths.

idea/src/org/jetbrains/jet/plugin/k2jsrun/K2JSRunConfigurationEditor.java

@@ -26,11 +26,18 @@ import com.intellij.openapi.options.SettingsEditor;
 import com.intellij.openapi.project.Project;
 import com.intellij.openapi.roots.ProjectRootManager;
 import com.intellij.openapi.ui.TextFieldWithBrowseButton;
+import com.jgoodies.looks.Fonts;
 import org.jetbrains.annotations.NotNull;
 
 import javax.swing.*;
+import javax.swing.event.DocumentEvent;
+import javax.swing.event.DocumentListener;
+import java.awt.*;
+import java.awt.event.ActionEvent;
+import java.awt.event.ActionListener;
 import java.awt.event.ItemEvent;
 import java.awt.event.ItemListener;
+import java.io.File;
 
 import static com.intellij.openapi.util.io.FileUtil.toSystemIndependentName;
 
@@ -88,6 +95,32 @@ public final class K2JSRunConfigurationEditor extends SettingsEditor<K2JSRunConf
             FileChooserDescriptorFactory.getDirectoryChooserDescriptor("directory where generated files will be stored");
         fileChooserDescriptor.setRoots(ProjectRootManager.getInstance(project).getContentRoots());
         generatedChooseFile.addBrowseFolderListener(null, null, project, fileChooserDescriptor);
+        final JTextField textField = generatedChooseFile.getTextField();
+        textField.getDocument().addDocumentListener(new DocumentListener() {
+            @Override
+            public void insertUpdate(DocumentEvent e) {
+                onChange();
+            }
+
+            @Override
+            public void removeUpdate(DocumentEvent e) {
+                onChange();
+            }
+
+            @Override
+            public void changedUpdate(DocumentEvent e) {
+                onChange();
+            }
+
+            private void onChange() {
+                File file = new File(generatedChooseFile.getText());
+                if (!file.isFile()) {
+                    textField.setForeground(Color.RED);
+                } else {
+                    textField.setForeground(Color.BLACK);
+                }
+            }
+        });
     }
 
     private void setUpShowInBrowserCheckBox() {