Make IndexableObjectKind parameterized.

pkg/analysis_server/lib/src/services/index/indexable_file.dart

Index: pkg/analysis_server/lib/src/services/index/indexable_file.dart
diff --git a/pkg/analysis_server/lib/src/services/index/indexable_file.dart b/pkg/analysis_server/lib/src/services/index/indexable_file.dart
index 36e3eeae96a213ba55d2f12b6b5a91e3525f1c71..7babfef4a5b73f9aa3ac82665ab5f231938d8373 100644
--- a/pkg/analysis_server/lib/src/services/index/indexable_file.dart
+++ b/pkg/analysis_server/lib/src/services/index/indexable_file.dart
@@ -42,7 +42,7 @@ class IndexableFile implements IndexableObject {
 /**
  * The kind of an indexable file.
  */
-class IndexableFileKind implements IndexableObjectKind {
+class IndexableFileKind implements IndexableObjectKind<IndexableFile> {
   /**
    * The unique instance of this class.
    */
@@ -62,13 +62,13 @@ class IndexableFileKind implements IndexableObjectKind {
   }
 
   @override
-  IndexableObject decode(AnalysisContext context, String filePath, int offset) {
+  IndexableFile decode(AnalysisContext context, String filePath, int offset) {
     return new IndexableFile(filePath);
   }
 
   @override
-  int encodeHash(StringToInt stringToInt, IndexableObject indexable) {
-    String path = (indexable as IndexableFile).path;
+  int encodeHash(StringToInt stringToInt, IndexableFile indexable) {
+    String path = indexable.path;
     return stringToInt(path);
   }
 }