Implement (partially) ?? being compile-time constant in analyzer.

pkg/analyzer/lib/src/generated/constant.dart

 // Copyright (c) 2014, 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 analyzer.src.generated.constant;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
@@ skipping 1598 matching lines @@
     }
     return result;
   }
 
   @override
   DartObjectImpl visitBinaryExpression(BinaryExpression node) {
     DartObjectImpl leftResult = node.leftOperand.accept(this);
     DartObjectImpl rightResult = node.rightOperand.accept(this);
     TokenType operatorType = node.operator.type;
     // 'null' is almost never good operand
-    if (operatorType != TokenType.BANG_EQ && operatorType != TokenType.EQ_EQ) {
+    if (operatorType != TokenType.BANG_EQ &&
+        operatorType != TokenType.EQ_EQ &&
+        operatorType != TokenType.QUESTION_QUESTION) {
       if (leftResult != null && leftResult.isNull ||
           rightResult != null && rightResult.isNull) {
         _error(node, CompileTimeErrorCode.CONST_EVAL_THROWS_EXCEPTION);
         return null;
       }
     }
     // evaluate operator
     while (true) {
       if (operatorType == TokenType.AMPERSAND) {
         return _dartObjectComputer.bitAnd(node, leftResult, rightResult);
@@ skipping 28 matching lines @@
       } else if (operatorType == TokenType.PERCENT) {
         return _dartObjectComputer.remainder(node, leftResult, rightResult);
       } else if (operatorType == TokenType.PLUS) {
         return _dartObjectComputer.add(node, leftResult, rightResult);
       } else if (operatorType == TokenType.STAR) {
         return _dartObjectComputer.times(node, leftResult, rightResult);
       } else if (operatorType == TokenType.SLASH) {
         return _dartObjectComputer.divide(node, leftResult, rightResult);
       } else if (operatorType == TokenType.TILDE_SLASH) {
         return _dartObjectComputer.integerDivide(node, leftResult, rightResult);
+      } else if (operatorType == TokenType.QUESTION_QUESTION) {
+        return _dartObjectComputer.questionQuestion(
+            node, leftResult, rightResult);
       } else {
         // TODO(brianwilkerson) Figure out which error to report.
         _error(node, null);
         return null;
       }
       break;
     }
   }
 
   @override
@@ skipping 622 matching lines @@
     if (leftOperand != null && rightOperand != null) {
       try {
         return leftOperand.equalEqual(_typeProvider, rightOperand);
       } on EvaluationException catch (exception) {
         _errorReporter.reportErrorForNode(exception.errorCode, node);
       }
     }
     return null;
   }
 
+  DartObjectImpl questionQuestion(Expression node, DartObjectImpl leftOperand,
+      DartObjectImpl rightOperand) {
+    if (leftOperand != null && rightOperand != null) {
+      if (leftOperand.isNull) {
+        return rightOperand;
+      }
+      return leftOperand;
+    }
+    return null;
+  }
+
   DartObjectImpl greaterThan(BinaryExpression node, DartObjectImpl leftOperand,
       DartObjectImpl rightOperand) {
     if (leftOperand != null && rightOperand != null) {
       try {
         return leftOperand.greaterThan(_typeProvider, rightOperand);
       } on EvaluationException catch (exception) {
         _errorReporter.reportErrorForNode(exception.errorCode, node);
       }
     }
     return null;
@@ skipping 3130 matching lines @@
       return BoolState.from(_element == rightElement);
     } else if (rightOperand is DynamicState) {
       return BoolState.UNKNOWN_VALUE;
     }
     return BoolState.FALSE_STATE;
   }
 
   @override
   String toString() => _element == null ? "-unknown-" : _element.name;
 }