analyzer strong mode: infer final field from initializer if super getters are dynamic

pkg/analyzer/lib/src/task/strong_mode.dart

Index: pkg/analyzer/lib/src/task/strong_mode.dart
diff --git a/pkg/analyzer/lib/src/task/strong_mode.dart b/pkg/analyzer/lib/src/task/strong_mode.dart
index f8cb97f6e8964224c15fbab6d0fdc2a9993c3bae..208521f061f47cae0a054108792b76974e723f67 100644
--- a/pkg/analyzer/lib/src/task/strong_mode.dart
+++ b/pkg/analyzer/lib/src/task/strong_mode.dart
@@ -294,10 +294,15 @@ class InstanceMemberInferrer {
         }
       }
       //
-      // Then, if none was found, infer the type from the initialization
-      // expression.
+      // If there is no overridden getter or if the overridden getter's type is
+      // dynamic, then we can infer the type from the initialization expression
+      // without breaking subtype rules. We could potentially infer a consistent
+      // return type even if the overridden getter's type was not dynamic, but
+      // choose not to for simplicity. The field is required to be final to
+      // prevent choosing a type that is inconsistent with assignments we cannot
+      // analyze.
       //
-      if (newType == null) {
+      if (newType == null || newType.isDynamic) {
         if (fieldElement.initializer != null &&
             (fieldElement.isFinal || overriddenGetters.isEmpty)) {
           newType = fieldElement.initializer.returnType;