Don't make assumptions about source encoding in analysis server tests (TBR).

pkg/analysis_server/test/analysis_server_test.dart

Index: pkg/analysis_server/test/analysis_server_test.dart
diff --git a/pkg/analysis_server/test/analysis_server_test.dart b/pkg/analysis_server/test/analysis_server_test.dart
index 64cb01042f4984c2697192c7af4851bfd0cf2a17..14713d9ecc6cff82f9f68121ca186e4323b46a64 100644
--- a/pkg/analysis_server/test/analysis_server_test.dart
+++ b/pkg/analysis_server/test/analysis_server_test.dart
@@ -85,7 +85,7 @@ class AnalysisServerTest {
           );
       expect(channel.notificationsReceived[1].event, equals('context.errors'));
       expect(channel.notificationsReceived[1].params['source'], equals(
-          '102file:///foo.dart')); // Issue 18739
+          source.encoding));
       List<AnalysisError> errors =
           channel.notificationsReceived[1].params['errors'];
       expect(errors, hasLength(1));