Make Comparable generic.

utils/css/source.dart

Index: utils/css/source.dart
diff --git a/utils/css/source.dart b/utils/css/source.dart
index 3875c7b5df571a21be363a808d103cc191811f6f..a5e981b69ac8b1cb46a9bb8f63ce7ba161daee70 100644
--- a/utils/css/source.dart
+++ b/utils/css/source.dart
@@ -6,7 +6,7 @@
 /**
  * Represents a file of source code.
  */
-class SourceFile implements Comparable {
+class SourceFile implements Comparable<SourceFile> {
   // TODO(terry): This filename for in memory buffer.  May need to rework if
   //              filename is used for more than informational.
   static String IN_MEMORY_FILE = '<buffer>';
@@ -132,7 +132,7 @@ class SourceFile implements Comparable {
  * work.
  */
  // TODO(jmesserly): Rename to Span - but first write cool refactoring tool
-class SourceSpan implements Comparable {
+class SourceSpan implements Comparable<SourceSpan> {
   /** The [SourceFile] that contains this span. */
   final SourceFile file;