Pass the 'strongMode' flag into SdkSummaryResynthesizer.

pkg/analyzer/lib/src/generated/sdk_io.dart

Index: pkg/analyzer/lib/src/generated/sdk_io.dart
diff --git a/pkg/analyzer/lib/src/generated/sdk_io.dart b/pkg/analyzer/lib/src/generated/sdk_io.dart
index 9bf52bf372fa44727b56b0e327fb129ba89ce2b0..09a7def0edf963b70529cd6f3e21a8806ed87244 100644
--- a/pkg/analyzer/lib/src/generated/sdk_io.dart
+++ b/pkg/analyzer/lib/src/generated/sdk_io.dart
@@ -284,8 +284,9 @@ class DirectoryBasedDartSdk implements DartSdk {
       if (_useSummary) {
         PackageBundle sdkBundle = getSummarySdkBundle();
         if (sdkBundle != null) {
-          _analysisContext.resultProvider =
-              new SdkSummaryResultProvider(_analysisContext, sdkBundle);
+          bool strongMode = _analysisOptions?.strongMode ?? false;
+          _analysisContext.resultProvider = new SdkSummaryResultProvider(
+              _analysisContext, sdkBundle, strongMode);
         }
       }
     }