Refactor qualified send sets.

pkg/compiler/lib/src/typechecker.dart

Index: pkg/compiler/lib/src/typechecker.dart
diff --git a/pkg/compiler/lib/src/typechecker.dart b/pkg/compiler/lib/src/typechecker.dart
index a2a52c14ca48e77f88bc4ac8433998a40c6b386f..b72eab124dacfcab5c1c9cb37cdf6541a74e108b 100644
--- a/pkg/compiler/lib/src/typechecker.dart
+++ b/pkg/compiler/lib/src/typechecker.dart
@@ -1041,6 +1041,10 @@ class TypeCheckerVisitor extends Visitor<DartType> {
       Element receiverElement = elements[node.receiver];
       if (receiverElement != null) {
         if (receiverElement.isPrefix) {
+          if (node.isConditional) {
+            // Skip cases like `prefix?.topLevel`.
+            return const DynamicAccess();
+          }
           assert(invariant(node, element != null,
               message: 'Prefixed node has no element.'));
           return computeResolvedAccess(node, name, element, memberKind);