Only set types of conditional expressions in strong mode.

pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart

Index: pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart b/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
index 5faf37545711d5ed2cb9da440701048008975739..1efc05f797d9bc72eb9666fa44ef0d924e07853e 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
@@ -664,7 +664,9 @@ abstract class TypeInferrerImpl extends TypeInferrer {
         listener.methodInvocationEnter(expression, typeContext) || typeNeeded;
     // First infer the receiver so we can look up the method that was invoked.
     var receiverType = inferExpression(receiver, null, true);
-    receiverVariable?.type = receiverType;
+    if (strongMode) {
+      receiverVariable?.type = receiverType;
+    }
     bool isOverloadedArithmeticOperator = false;
     Member interfaceMember =
         findMethodInvocationMember(receiverType, desugaredInvocation);
@@ -719,7 +721,9 @@ abstract class TypeInferrerImpl extends TypeInferrer {
         listener.propertyGetEnter(expression, typeContext) || typeNeeded;
     // First infer the receiver so we can look up the getter that was invoked.
     var receiverType = inferExpression(receiver, null, true);
-    receiverVariable?.type = receiverType;
+    if (strongMode) {
+      receiverVariable?.type = receiverType;
+    }
     Member interfaceMember =
         findInterfaceMember(receiverType, desugaredGet.name, fileOffset);
     if (isTopLevel &&