[analyzer] Fix broken code generators in g3.

Some part of the code generators trigger the `??` RHS operand when there is an unresolved type on the LHS.

In the previous const evaluator prior to https://dart-review.googlesource.com/c/sdk/+/312347, errors triggered by unresolved types would be ignored. Now that we have specific unresolved constants, we need to short circuit the evaluation in some places.

Anyways, this change makes unresolved types return LHS early (they're represented by a `Null` state) until I can find a better fix for this.

b/293326927

Change-Id: Ifcbff9921e1d4a82dc24153b042eaaa862bcca68
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/316490
Commit-Queue: Kallen Tu <kallentu@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/dart/constant/evaluation.dart

@@ -2023,6 +2023,11 @@ class DartObjectComputer {
   Constant lazyQuestionQuestion(Expression node, DartObjectImpl leftOperand,
       Constant Function() rightOperandComputer) {
     if (leftOperand.isNull) {
+      // TODO(kallentu): Remove this once we have a better representation for
+      // unresolved types.
+      if (leftOperand.isInvalid) {
+        return leftOperand;
+      }
       return rightOperandComputer();
     }
     return leftOperand;