Turn off `strong_mode_down_cast_composite` warnings by default in Flutter projects.

pkg/analyzer/lib/src/context/builder.dart

Index: pkg/analyzer/lib/src/context/builder.dart
diff --git a/pkg/analyzer/lib/src/context/builder.dart b/pkg/analyzer/lib/src/context/builder.dart
index 2d8b0f14a5ef548f6428bd5219fb070827cc96ab..62b700d3234b2711f419df0ffb1ca7e76fe334b6 100644
--- a/pkg/analyzer/lib/src/context/builder.dart
+++ b/pkg/analyzer/lib/src/context/builder.dart
@@ -388,7 +388,7 @@ class ContextBuilder {
       // Search for the default analysis options
       Source source;
       // TODO(danrubel) determine if bazel or gn project depends upon flutter
-      if (workspace.hasFlutterDependency) {
+      if (sourceFactory.hasFlutterDependency) {
         source =
             sourceFactory.forUri('package:flutter/analysis_options_user.yaml');
       }
@@ -730,10 +730,6 @@ class _BasicWorkspace extends Workspace {
   _BasicWorkspace._(this.provider, this.root, this._builder);
 
   @override
-  // Alternately, we could check the pubspec for "sdk: flutter"
-  bool get hasFlutterDependency => packageMap.containsKey('flutter');
-
-  @override
   Map<String, List<Folder>> get packageMap {
     _packageMap ??= _builder.convertPackagesToMap(packages);
     return _packageMap;