Revert use of ?? to unbreak the build

pkg/compiler/lib/src/dump_info.dart

 // Copyright (c) 2013, 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 dump_info;
 
 import 'dart:convert'
     show HtmlEscape, JsonEncoder, StringConversionSink, ChunkedConversionSink;
 
 import 'package:dart2js_info/info.dart';
@@ skipping 453 matching lines @@
   // was generated for an element.  If no source
   // code was produced, return 0.
   int sizeOf(Element element) {
     if (_elementToNodes.containsKey(element)) {
       return _elementToNodes[element].map(sizeOfNode).fold(0, (a, b) => a + b);
     } else {
       return 0;
     }
   }
 
-  int sizeOfNode(jsAst.Node node) => _nodeToSize[node] ?? 0;
+  int sizeOfNode(jsAst.Node node) {
+    // TODO(sigmund): switch back to null aware operators (issue #24136)
+    var size = _nodeToSize[node];
+    return size == null ? 0 : size;
+  }
 
   String codeOf(Element element) {
     List<jsAst.Node> code = _elementToNodes[element];
     if (code == null) return null;
     // Concatenate rendered ASTs.
     StringBuffer sb = new StringBuffer();
     for (jsAst.Node ast in code) {
       sb.writeln(jsAst.prettyPrint(ast, compiler).getText());
     }
     return sb.toString();
@@ skipping 73 matching lines @@
 
     ChunkedConversionSink<Object> sink = encoder.startChunkedConversion(
         new StringConversionSink.fromStringSink(buffer));
     sink.add(result.toJson());
     compiler.reportInfo(NO_LOCATION_SPANNABLE, MessageKind.GENERIC, {
       'text': "View the dumped .info.json file at "
           "https://dart-lang.github.io/dump-info-visualizer"
     });
   }
 }