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

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 a56197896785725423da2add3721501023f16a5c..ce6ed09477528f9feb0257ddebcb9a17a5f13394 100644
--- a/pkg/analyzer/lib/src/generated/constant.dart
+++ b/pkg/analyzer/lib/src/generated/constant.dart
@@ -1616,7 +1616,9 @@ 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);
@@ -1665,6 +1667,9 @@ 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);
@@ -2307,6 +2312,17 @@ 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) {