Fix 'MockContext does not implement getContents' test failures.

pkg/analysis_server/test/domain_completion_test.dart

Index: pkg/analysis_server/test/domain_completion_test.dart
diff --git a/pkg/analysis_server/test/domain_completion_test.dart b/pkg/analysis_server/test/domain_completion_test.dart
index f6650cf7cfd7d8be4a812be8116a184c59ecab74..ded3d0d0d5eaec51040ace78f7e2a84a94f30e51 100644
--- a/pkg/analysis_server/test/domain_completion_test.dart
+++ b/pkg/analysis_server/test/domain_completion_test.dart
@@ -547,6 +547,11 @@ class MockContext implements AnalysisContext {
   @override
   Stream<SourcesChangedEvent> get onSourcesChanged => mockStream;
 
+  @override
+  TimestampedData<String> getContents(Source source) {
+    return source.contents;
+  }
+
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }