Only set types of conditional expressions in strong mode.

pkg/front_end/lib/src/fasta/kernel/frontend_accessors.dart

Index: pkg/front_end/lib/src/fasta/kernel/frontend_accessors.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/frontend_accessors.dart b/pkg/front_end/lib/src/fasta/kernel/frontend_accessors.dart
index a894a3a6a3fec3630720b5f18b039607ee724982..669e0f726575a9e28f8d1cd46a91015b61aaabb3 100644
--- a/pkg/front_end/lib/src/fasta/kernel/frontend_accessors.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/frontend_accessors.dart
@@ -348,10 +348,7 @@ class NullAwarePropertyAccessor extends Accessor {
       Expression body, KernelComplexAssignment complexAssignment) {
     var offset = offsetForToken(token);
     var nullAwareGuard = new ConditionalExpression(
-        buildIsNull(receiverAccess(), offset),
-        new NullLiteral(),
-        body,
-        const DynamicType())
+        buildIsNull(receiverAccess(), offset), new NullLiteral(), body, null)
       ..fileOffset = offset;
     if (complexAssignment != null) {
       body = makeLet(receiver, nullAwareGuard);