Make ?? a compile-time constant operator.

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

Index: pkg/analyzer/lib/src/generated/constant.dart
diff --git a/pkg/analyzer/lib/src/generated/constant.dart b/pkg/analyzer/lib/src/generated/constant.dart
index 24125064171085e4cfad5f3679f3da5839bd62cb..c4d7027ded91d9fa85af5c4a28d96f224fce8710 100644
--- a/pkg/analyzer/lib/src/generated/constant.dart
+++ b/pkg/analyzer/lib/src/generated/constant.dart
@@ -1533,7 +1533,9 @@ class ConstantVisitor extends UnifyingAstVisitor<DartObjectImpl> {
     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);
@@ -1582,6 +1584,9 @@ class ConstantVisitor extends UnifyingAstVisitor<DartObjectImpl> {
         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);
@@ -2295,6 +2300,15 @@ class DartObjectComputer {
     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) {