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

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,
         KernelComplexAssignment,
@@ skipping 259 matching lines @@
   @override
   Expression buildPostfixIncrement(Name binaryOperator,
       {int offset: TreeNode.noOffset,
       bool voidContext: false,
       Procedure interfaceTarget}) {
     return buildError(new KernelArguments(<Expression>[new IntLiteral(1)]),
         isGetter: true);
   }
 
   @override
-  Expression buildNullAwareAssignment(Expression value, DartType type,
+  Expression buildNullAwareAssignment(
+      Expression value, DartType type, int offset,
       {bool voidContext: false}) {
     return buildError(new KernelArguments(<Expression>[value]), isSetter: true);
   }
 
   @override
   Expression buildSimpleRead() =>
       buildError(new Arguments.empty(), isGetter: true);
 
   @override
   Expression makeInvalidRead() =>
@@ skipping 95 matching lines @@
       return helper.buildSuperInitializer(constructor, arguments, offset);
     } else {
       return helper.buildRedirectingInitializer(constructor, arguments, offset);
     }
   }
 
   Expression buildAssignment(Expression value, {bool voidContext: false}) {
     return buildAssignmentError();
   }
 
-  Expression buildNullAwareAssignment(Expression value, DartType type,
+  Expression buildNullAwareAssignment(
+      Expression value, DartType type, int offset,
       {bool voidContext: false}) {
     return buildAssignmentError();
   }
 
   Expression buildCompoundAssignment(Name binaryOperator, Expression value,
       {int offset: TreeNode.noOffset,
       bool voidContext: false,
       Procedure interfaceTarget,
       bool isPreIncDec: false}) {
     return buildAssignmentError();
@@ skipping 90 matching lines @@
       }
       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,
+  Expression buildNullAwareAssignment(
+      Expression value, DartType type, int offset,
       {bool voidContext: false}) {
     return internalError("Unhandled");
   }
 
   Expression buildCompoundAssignment(Name binaryOperator, Expression value,
       {int offset,
       bool voidContext: false,
       Procedure interfaceTarget,
       bool isPreIncDec: false}) {
     return internalError("Unhandled");
@@ skipping 44 matching lines @@
             "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,
+  Expression buildNullAwareAssignment(
+      Expression value, DartType type, int offset,
       {bool voidContext: false}) {
     return internalError("Unhandled");
   }
 
   Expression buildCompoundAssignment(Name binaryOperator, Expression value,
       {int offset,
       bool voidContext: false,
       Procedure interfaceTarget,
       bool isPreIncDec: false}) {
     return internalError("Unhandled");
@@ skipping 450 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);
 }