Add offsets to the `== null` calls inserted by null-aware desugaring.

pkg/front_end/lib/src/fasta/testing/validating_instrumentation.dart

 // Copyright (c) 2017, 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.md file.
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:front_end/src/base/instrumentation.dart';
 import 'package:front_end/src/fasta/messages.dart';
 import 'package:front_end/src/fasta/scanner.dart';
@@ skipping 138 matching lines @@
           }
         }
       }
     }
   }
 
   @override
   void record(
       Uri uri, int offset, String property, InstrumentationValue value) {
     uri = Uri.base.resolveUri(uri);
+    if (offset == -1) {
+      throw _formatProblem(uri, 0, 'No offset for $property=$value', null);
+    }
     var expectationsForUri = _unsatisfiedExpectations[uri];
     if (expectationsForUri == null) return;
     var expectationsAtOffset = expectationsForUri[offset];
     if (expectationsAtOffset != null) {
       for (int i = 0; i < expectationsAtOffset.length; i++) {
         var expectation = expectationsAtOffset[i];
         if (expectation.property == property) {
           if (!value.matches(expectation.value)) {
             _problemWithStack(
                 uri,
@@ skipping 69 matching lines @@
       this.property, this.value, this.commentOffset, this.commentLength);
 }
 
 class _Fix {
   final int offset;
   final int length;
   final String replacement;
 
   _Fix(this.offset, this.length, this.replacement);
 }