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

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 779 matching lines @@
     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(
         variable,
-        new KernelConditionalExpression(buildIsNull(new VariableGet(variable)),
-            b, new VariableGet(variable))));
+        new KernelConditionalExpression(
+            buildIsNull(new VariableGet(variable), offsetForToken(token)),
+            b,
+            new VariableGet(variable))));
   }
 
   /// Handle `a?.b(...)`.
   void doIfNotNull(Token token) {
     IncompleteSend send = pop();
     push(send.withReceiver(pop(), token.charOffset, isNullAware: true));
   }
 
   void doDotOrCascadeExpression(Token token) {
     // TODO(ahe): Handle null-aware.
@@ skipping 2262 matching lines @@
       bool isNullAware: false,
       bool isImplicitCall: false}) {
     if (constantExpressionRequired && !isConstantExpression) {
       return buildCompileTimeError("Not a constant expression.", offset);
     }
     if (isNullAware) {
       VariableDeclaration variable = new VariableDeclaration.forValue(receiver);
       return makeLet(
           variable,
           new KernelConditionalExpression(
-              buildIsNull(new VariableGet(variable)),
+              buildIsNull(new VariableGet(variable), offset),
               new NullLiteral(),
               new MethodInvocation(new VariableGet(variable), name, arguments)
                 ..fileOffset = offset));
     } else {
       return new KernelMethodInvocation(receiver, name, arguments,
           isImplicitCall: isImplicitCall)
         ..fileOffset = offset;
     }
   }
 
@@ skipping 67 matching lines @@
   }
 
   Expression buildSimpleRead();
 
   Expression buildForEffect();
 
   Expression buildAssignment(Expression value, {bool voidContext: false}) {
     return makeInvalidWrite(value);
   }
 
-  Expression buildNullAwareAssignment(Expression value, DartType type,
+  Expression buildNullAwareAssignment(
+      Expression value, DartType type, int offset,
       {bool voidContext: false}) {
     return makeInvalidWrite(value);
   }
 
   Expression buildCompoundAssignment(Name binaryOperator, Expression value,
       {int offset: TreeNode.noOffset,
       bool voidContext: false,
       Procedure interfaceTarget,
       bool isPreIncDec: false}) {
     return makeInvalidWrite(value);
@@ skipping 64 matching lines @@
     } else if (identical("/=", assignmentOperator)) {
       return accessor.buildCompoundAssignment(divisionName, value,
           offset: offsetForToken(token), voidContext: voidContext);
     } else if (identical("<<=", assignmentOperator)) {
       return accessor.buildCompoundAssignment(leftShiftName, value,
           offset: offsetForToken(token), voidContext: voidContext);
     } else if (identical(">>=", assignmentOperator)) {
       return accessor.buildCompoundAssignment(rightShiftName, value,
           offset: offsetForToken(token), voidContext: voidContext);
     } else if (identical("??=", assignmentOperator)) {
-      return accessor.buildNullAwareAssignment(value, const DynamicType(),
+      return accessor.buildNullAwareAssignment(
+          value, const DynamicType(), offsetForToken(token),
           voidContext: voidContext);
     } else if (identical("^=", assignmentOperator)) {
       return accessor.buildCompoundAssignment(caretName, value,
           offset: offsetForToken(token), voidContext: voidContext);
     } else if (identical("|=", assignmentOperator)) {
       return accessor.buildCompoundAssignment(barName, value,
           offset: offsetForToken(token), voidContext: voidContext);
     } else if (identical("~/=", assignmentOperator)) {
       return accessor.buildCompoundAssignment(mustacheName, value,
           offset: offsetForToken(token), voidContext: voidContext);
@@ skipping 283 matching lines @@
     if (starToken == null) {
       return AsyncMarker.Async;
     } else {
       assert(identical(starToken.stringValue, "*"));
       return AsyncMarker.AsyncStar;
     }
   } else {
     return internalError("Unknown async modifier: $asyncToken");
   }
 }