ignore unreachable constructors in reflective tests

These were flagged as unused by `unreachable_from_main` but they are
used by `dart:mirrors`.

Change-Id: I3637b4b234f0bf7fe795977cb7ec6a1ca5449810
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/293006
Reviewed-by: Phil Quitslund <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: srawlins

pkg/analysis_server/test/services/snippets/snippet_request_test.dart

@@ -19,7 +19,9 @@ void main() {
 
 @reflectiveTest
 class SnippetRequestTest extends AbstractSingleUnitTest {
-  SnippetRequestTest() {
+  @override
+  void setUp() {
+    super.setUp();
     verifyNoTestUnitErrors = false;
   }
 

pkg/nnbd_migration/test/decorated_type_test.dart

@@ -37,6 +37,7 @@ class DecoratedTypeTest extends Object
   @override
   final decoratedTypeParameterBounds = DecoratedTypeParameterBounds();
 
+  // ignore: unreachable_from_main
   factory DecoratedTypeTest() {
     var typeProvider = TestTypeProvider();
     var graph = NullabilityGraph();

pkg/nnbd_migration/test/edge_builder_test.dart

@@ -60,6 +60,7 @@ class AssignmentCheckerTest extends Object
 
   final AssignmentCheckerForTesting checker;
 
+  // ignore: unreachable_from_main
   factory AssignmentCheckerTest() {
     var typeProvider = TestTypeProvider().asLegacy;
     _setCoreLibrariesTypeSystem(typeProvider);