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

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.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 file.
 
 /// This file declares a "shadow hierarchy" of concrete classes which extend
 /// the kernel class hierarchy, adding methods and fields needed by the
 /// BodyBuilder.
 ///
 /// Instances of these classes may be created using the factory methods in
 /// `ast_factory.dart`.
@@ skipping 310 matching lines @@
     if (isPostIncDec) parts.add('isPostIncDec=true');
     if (isPreIncDec) parts.add('isPreIncDec=true');
     return parts;
   }
 
   DartType _inferRhs(KernelTypeInferrer inferrer, DartType writeContext) {
     DartType inferredType = writeContext ?? const DynamicType();
     if (nullAwareCombiner != null) {
       var rhsType = inferrer.inferExpression(rhs, writeContext, true);
       MethodInvocation equalsInvocation = nullAwareCombiner.condition;
-      inferrer.findMethodInvocationMember(writeContext, equalsInvocation);
+      inferrer.findMethodInvocationMember(writeContext, equalsInvocation,
+          silent: true);
       return inferrer.typeSchemaEnvironment
           .getLeastUpperBound(inferredType, rhsType);
     } else if (combiner != null) {
       bool isOverloadedArithmeticOperator = false;
       var combinerMember = inferrer
           .findMethodInvocationMember(writeContext, combiner, silent: true);
       if (combinerMember is Procedure) {
         isOverloadedArithmeticOperator = inferrer.typeSchemaEnvironment
             .isOverloadedArithmeticOperator(combinerMember);
       }
@@ skipping 1851 matching lines @@
   }
 
   transformChildren(v) {
     return internalError("Internal error: Unsupported operation.");
   }
 
   visitChildren(v) {
     return internalError("Internal error: Unsupported operation.");
   }
 }