Add analysis option that will be used to fix #26583

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

Index: pkg/analyzer/lib/src/generated/engine.dart
diff --git a/pkg/analyzer/lib/src/generated/engine.dart b/pkg/analyzer/lib/src/generated/engine.dart
index 6d41c4fb044afd87ebad885f5740ee7c495735ed..2c4c4df8cfae9243efb4fae7aad6a98393b51604 100644
--- a/pkg/analyzer/lib/src/generated/engine.dart
+++ b/pkg/analyzer/lib/src/generated/engine.dart
@@ -1284,6 +1284,14 @@ class AnalysisOptionsImpl implements AnalysisOptions {
   bool strongModeHints = false;
 
   /**
+   * A flag indicating whether implicit casts are allowed in [strongMode]
+   * (they are always allowed in Dart 1.0 mode).
+   *
+   * This option is experimental and subject to change.
+   */
+  bool implicitCasts = true;
+
+  /**
    * Initialize a newly created set of analysis options to have their default
    * values.
    */
@@ -1314,6 +1322,7 @@ class AnalysisOptionsImpl implements AnalysisOptions {
     strongMode = options.strongMode;
     if (options is AnalysisOptionsImpl) {
       strongModeHints = options.strongModeHints;
+      implicitCasts = options.implicitCasts;
     }
   }