Revert "Clean up the use of deprecated API in the analyzer_plugin package"

This reverts commit 86ba29265a.

The replacement API (int.tryParse) has only just been introduced and
is not yet avaiable to internal Google users.  I plan to un-do the
commit in about a week once the internal SDK has been updated.

Change-Id: Ic9206231861400d42f4814e4a56bde57ba8705a5
Reviewed-on: https://dart-review.googlesource.com/52822
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/analyzer_plugin/lib/src/protocol/protocol_internal.dart

@@ -277,11 +277,9 @@ abstract class JsonDecoder {
     if (json is int) {
       return json;
     } else if (json is String) {
-      int value = int.tryParse(json);
-      if (value == null) {
+      return int.parse(json, onError: (String value) {
         throw mismatch(jsonPath, 'int', json);
-      }
-      return value;
+      });
     }
     throw mismatch(jsonPath, 'int', json);
   }