add UNKNOWN_SOURCE error code and remove 2 unused legacy error codes

pkg/analysis_server/test/integration/protocol_matchers.dart

Index: pkg/analysis_server/test/integration/protocol_matchers.dart
diff --git a/pkg/analysis_server/test/integration/protocol_matchers.dart b/pkg/analysis_server/test/integration/protocol_matchers.dart
index 1909ee9ece6a8721ff4064bd43c042259bc35d70..7e0500e0c473cca1b5a84350800266a0a4c9d95a 100644
--- a/pkg/analysis_server/test/integration/protocol_matchers.dart
+++ b/pkg/analysis_server/test/integration/protocol_matchers.dart
@@ -1833,6 +1833,7 @@ final Matcher isRequestError = new LazyMatcher(() => new MatchesJsonObject(
  *   SORT_MEMBERS_PARSE_ERRORS
  *   UNANALYZED_PRIORITY_FILES
  *   UNKNOWN_REQUEST
+ *   UNKNOWN_SOURCE
  *   UNSUPPORTED_FEATURE
  * }
  */
@@ -1852,6 +1853,7 @@ final Matcher isRequestErrorCode = new MatchesEnum("RequestErrorCode", [
   "SORT_MEMBERS_PARSE_ERRORS",
   "UNANALYZED_PRIORITY_FILES",
   "UNKNOWN_REQUEST",
+  "UNKNOWN_SOURCE",
   "UNSUPPORTED_FEATURE"
 ]);