[vm/aot/tfa] More robust handling of annotations in TFA

Apparently it is possible for annotations to contain InvalidExpression
AST nodes, not just ConstantExpression
(see flutter/flutter#83466 (comment)).

This change improves robustness of TFA by ignoring (and cleaning) all
annotations which are not ConstantExpression, instead of crashing on
them.

TEST=none
(I wasn't able to reproduce the problem and replicate the situation
where front-end generates InvalidExpression nodes.)

Change-Id: I1dbd55515f1e861488f6cc46eea50a2ef31ab564
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/203283
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Alexander Markov <[email protected]>

Author: alexmarkov

pkg/vm/lib/transformations/type_flow/transformer.dart

@@ -154,12 +154,15 @@ class CleanupAnnotations extends RecursiveVisitor {
   }
 
   bool _keepAnnotation(Expression annotation) {
-    final constant = (annotation as ConstantExpression).constant;
-    if (constant is InstanceConstant) {
-      final cls = constant.classNode;
-      return (cls == externalNameClass) ||
-          (cls == pragmaClass) ||
-          (protobufHandler != null && protobufHandler.usesAnnotationClass(cls));
+    if (annotation is ConstantExpression) {
+      final constant = annotation.constant;
+      if (constant is InstanceConstant) {
+        final cls = constant.classNode;
+        return (cls == externalNameClass) ||
+            (cls == pragmaClass) ||
+            (protobufHandler != null &&
+                protobufHandler.usesAnnotationClass(cls));
+      }
     }
     return false;
   }