Complain about incorrect use of null-aware operations.

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.fasta_accessors;
 
 import 'package:front_end/src/fasta/kernel/kernel_shadow_ast.dart'
     show KernelArguments, KernelThisExpression;
 
 import 'package:front_end/src/fasta/kernel/utils.dart' show offsetForToken;
@@ skipping 303 matching lines @@
     int offset = offsetForToken(token);
     return helper.buildInvalidInitializer(
         helper.buildCompileTimeError(
             "Can't use '$keyword' here, did you mean '$keyword()'?", offset),
         offset);
   }
 
   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);
     }
     if (send is SendAccessor) {
       // Notice that 'this' or 'super' can't be null. So we can ignore the
       // value of [isNullAware].
       MethodInvocation result = helper.buildMethodInvocation(
           new KernelThisExpression(),
           send.name,
           send.arguments,
@@ skipping 132 matching lines @@
   Expression buildAssignment(Expression value, {bool voidContext: false}) {
     return internalError("Unhandled");
   }
 
   withReceiver(Object receiver, int operatorOffset, {bool isNullAware: false}) {
     if (receiver is FastaAccessor) {
       return receiver.buildPropertyAccess(this, operatorOffset, isNullAware);
     }
     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));
     }
     return helper.buildMethodInvocation(
         helper.toValue(receiver), name, arguments, offsetForToken(token),
         isNullAware: isNullAware);
   }
 
   Expression buildNullAwareAssignment(Expression value, DartType type,
@@ skipping 37 matching lines @@
   Expression buildAssignment(Expression value, {bool voidContext: false}) {
     return internalError("Unhandled");
   }
 
   withReceiver(Object receiver, int operatorOffset, {bool isNullAware: false}) {
     if (receiver is FastaAccessor) {
       return receiver.buildPropertyAccess(this, operatorOffset, isNullAware);
     }
     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);
     }
 
     return PropertyAccessor.make(
         helper, token, helper.toValue(receiver), name, null, null, isNullAware);
   }
 
   Expression buildNullAwareAssignment(Expression value, DartType type,
       {bool voidContext: false}) {
@@ skipping 428 matching lines @@
     offset ??= offsetForToken(this.token);
     return helper.throwNoSuchMethodError(new NullLiteral()..fileOffset = offset,
         plainNameForRead, arguments, offset,
         isGetter: isGetter, isSetter: isSetter);
   }
 }
 
 bool isFieldOrGetter(Member member) {
   return member is Field || (member is Procedure && member.isGetter);
 }