Complain about incorrect use of null-aware operations.

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

Index: pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart b/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
index e9dadd785fde5098cf96b6d54c24038d4daca17e..dabc038ae6e826462398cef27a812fd597e71b8f 100644
--- a/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
@@ -321,6 +321,10 @@ class ThisAccessor extends FastaAccessor {
   buildPropertyAccess(
       IncompleteSend send, int operatorOffset, bool isNullAware) {
     if (isInitializer && send is SendAccessor) {
+      if (isNullAware) {
+        helper.addCompileTimeError(
+            operatorOffset, "Expected '.'\nTry removing '?'.");
+      }
       return buildConstructorInitializer(
           offsetForToken(send.token), send.name, send.arguments);
     }
@@ -473,6 +477,12 @@ class SendAccessor extends IncompleteSend {
     }
     if (receiver is PrefixBuilder) {
       PrefixBuilder prefix = receiver;
+      if (isNullAware) {
+        helper.addCompileTimeError(
+            offsetForToken(token),
+            "Library prefix '${prefix.name}' can't be used with null-aware "
+            "operator.\nTry removing '?'.");
+      }
       receiver = helper.scopeLookup(prefix.exports, name.name, token,
           isQualified: true, prefix: prefix);
       return helper.finishSend(receiver, arguments, offsetForToken(token));
@@ -530,6 +540,12 @@ class IncompletePropertyAccessor extends IncompleteSend {
     }
     if (receiver is PrefixBuilder) {
       PrefixBuilder prefix = receiver;
+      if (isNullAware) {
+        helper.addCompileTimeError(
+            offsetForToken(token),
+            "Library prefix '${prefix.name}' can't be used with null-aware "
+            "operator.\nTry removing '?'.");
+      }
       return helper.scopeLookup(prefix.exports, name.name, token,
           isQualified: true, prefix: prefix);
     }