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

pkg/analyzer/lib/src/task/strong/checker.dart

Index: pkg/analyzer/lib/src/task/strong/checker.dart
diff --git a/pkg/analyzer/lib/src/task/strong/checker.dart b/pkg/analyzer/lib/src/task/strong/checker.dart
index 491b50a6703913089c0f861a7cca12038e01a1b4..b9918a0e8390f53ff9fdfe28bbe61c18ba4e3a84 100644
--- a/pkg/analyzer/lib/src/task/strong/checker.dart
+++ b/pkg/analyzer/lib/src/task/strong/checker.dart
@@ -147,13 +147,14 @@ class CodeChecker extends RecursiveAstVisitor {
   final TypeProvider typeProvider;
   final AnalysisErrorListener reporter;
   final AnalysisOptionsImpl _options;
+  final bool _hasFlutterDependency;
   _OverrideChecker _overrideChecker;
 
   bool _failure = false;
   bool _hasImplicitCasts;
 
   CodeChecker(TypeProvider typeProvider, StrongTypeSystemImpl rules,
-      AnalysisErrorListener reporter, this._options)
+      AnalysisErrorListener reporter, this._options, this._hasFlutterDependency)
       : typeProvider = typeProvider,
         rules = rules,
         reporter = reporter {
@@ -1111,6 +1112,17 @@ class CodeChecker extends RecursiveAstVisitor {
       }
     }
 
+    // BUG(28588): The down-cast-composite warning will be turned off by
+    // default, but as a first step we're only doing this for Flutter code
+    // and we will leave it on for web projects to help with the migration
+    // to strong mode and the associate runtime type checks. Flutter projects
+    // can still turn on the warning explicitly through analysis options.
+    if (_hasFlutterDependency && downCastComposite) {
+      bool hasExplicitOption = _options.errorProcessors.any(
+          (processor) => processor.code == StrongModeCode.DOWN_CAST_COMPOSITE);
+      if (!hasExplicitOption) return;
+    }
+
     var parent = expr.parent;
     ErrorCode errorCode;
     if (downCastComposite) {