Implement type inference for if-null expressions (a ?? b).

pkg/front_end/lib/src/fasta/kernel/body_builder.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.body_builder;
 
 import '../fasta_codes.dart'
     show
         FastaMessage,
         codeConstFieldWithoutInitializer,
@@ skipping 846 matching lines @@
     Expression argument = popForValue();
     Expression receiver = popForValue();
     push(new KernelLogicalExpression(receiver, token.stringValue, argument));
   }
 
   /// Handle `a ?? b`.
   void doIfNull(Token token) {
     Expression b = popForValue();
     Expression a = popForValue();
     VariableDeclaration variable = new VariableDeclaration.forValue(a);
-    push(makeLet(
+    push(new KernelIfNullExpression(
         variable,
         new KernelConditionalExpression(
             buildIsNull(new VariableGet(variable), offsetForToken(token)),
             b,
             new VariableGet(variable))));
   }
 
   /// Handle `a?.b(...)`.
   void doIfNotNull(Token token) {
     IncompleteSend send = pop();
@@ skipping 2792 matching lines @@
     if (starToken == null) {
       return AsyncMarker.Async;
     } else {
       assert(identical(starToken.stringValue, "*"));
       return AsyncMarker.AsyncStar;
     }
   } else {
     return internalError("Unknown async modifier: $asyncToken");
   }
 }