Changed abstract class to concrete class.

List<T> is an interface and can't be instantiated.
ArrayList<T> is a concrete class and implements the interface List<T>.

Author: Chris--A

arduino-core/src/cc/arduino/contributions/libraries/LibrariesIndexer.java

@@ -58,7 +58,7 @@ public class LibrariesIndexer {
   private final File stagingFolder;
   private File sketchbookLibrariesFolder;
 
-  private static final List<String> badLibNotified = new List<String>();
+  private static final List<String> badLibNotified = new ArrayList<String>();
 
   public LibrariesIndexer(File preferencesFolder) {
     indexFile = new File(preferencesFolder, "library_index.json");