Simplify summary resynthesis tests

By removing unnecessary flags isStrongMode and isSharedFrontEnd.

Change-Id: I18ebb15297cc7dde38d61369d9a18b9e3bce895a
Reviewed-on: https://dart-review.googlesource.com/74966
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/analyzer/test/src/summary/resynthesize_ast_test.dart

@@ -258,13 +258,9 @@ abstract class _ResynthesizeAstTest extends ResynthesizeTest
 
   @override
   AnalysisOptionsImpl createOptions() {
-    if (isStrongMode) {
-      return super.createOptions()
-        ..previewDart2 = true
-        ..isMixinSupportEnabled = true;
-    } else {
-      return super.createOptions()..previewDart2 = false;
-    }
+    return super.createOptions()
+      ..previewDart2 = true
+      ..isMixinSupportEnabled = true;
   }
 
   test_getElement_constructor_named() async {