Support + concatenation in constant expressions with strings

pkg/analyzer/lib/src/dart/ast/utilities.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.dart.ast.utilities;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_ast_factory.dart';
@@ skipping 2172 matching lines @@
  * >   </span>
  * > * <span>
  * >   An expression of one of the forms <i>~e</i>, <i>e<sub>1</sub> ^
  * >   e<sub>2</sub></i>, <i>e<sub>1</sub> &amp; e<sub>2</sub></i>,
  * >   <i>e<sub>1</sub> | e<sub>2</sub></i>, <i>e<sub>1</sub> &gt;&gt;
  * >   e<sub>2</sub></i> or <i>e<sub>1</sub> &lt;&lt; e<sub>2</sub></i>, where
  * >   <i>e</i>, <i>e<sub>1</sub></i> and <i>e<sub>2</sub></i> are constant
  * >   expressions that evaluate to an integer value or to <b>null</b>.
  * >   </span>
  * > * <span>
- * >   An expression of one of the forms <i>-e</i>, <i>e<sub>1</sub> +
- * >   e<sub>2</sub></i>, <i>e<sub>1</sub> -e<sub>2</sub></i>,
- * >   <i>e<sub>1</sub> * e<sub>2</sub></i>, <i>e<sub>1</sub> /
- * >   e<sub>2</sub></i>, <i>e<sub>1</sub> ~/ e<sub>2</sub></i>,
- * >   <i>e<sub>1</sub> &gt; e<sub>2</sub></i>, <i>e<sub>1</sub> &lt;
- * >   e<sub>2</sub></i>, <i>e<sub>1</sub> &gt;= e<sub>2</sub></i>,
- * >   <i>e<sub>1</sub> &lt;= e<sub>2</sub></i> or <i>e<sub>1</sub> %
- * >   e<sub>2</sub></i>, where <i>e</i>, <i>e<sub>1</sub></i> and
- * >   <i>e<sub>2</sub></i> are constant expressions that evaluate to a numeric
- * >   value or to <b>null</b>.
+ * >   An expression of one of the forms <i>-e</i>, <i>e<sub>1</sub>
+ * >   -e<sub>2</sub></i>, <i>e<sub>1</sub> * e<sub>2</sub></i>,
+ * >   <i>e<sub>1</sub> / e<sub>2</sub></i>, <i>e<sub>1</sub> ~/
+ * >   e<sub>2</sub></i>, <i>e<sub>1</sub> &gt; e<sub>2</sub></i>,
+ * >   <i>e<sub>1</sub> &lt; e<sub>2</sub></i>, <i>e<sub>1</sub> &gt;=
+ * >   e<sub>2</sub></i>, <i>e<sub>1</sub> &lt;= e<sub>2</sub></i> or
+ * >   <i>e<sub>1</sub> % e<sub>2</sub></i>, where <i>e</i>,
+ * >   <i>e<sub>1</sub></i> and <i>e<sub>2</sub></i> are constant expressions
+ * >   that evaluate to a numeric value or to <b>null</b>.
+ * >   </span>
+ * > * <span>
+ * >   An expression of one the form <i>e<sub>1</sub> + e<sub>2</sub></i>,
+ * >   <i>e<sub>1</sub> -e<sub>2</sub></i> where <i>e<sub>1</sub> and
+ * >   e<sub>2</sub></i> are constant expressions that evaluate to a numeric or
+ * >   string value or to <b>null</b>.
  * >   </span>
  * > * <span>
  * >   An expression of the form <i>e<sub>1</sub> ? e<sub>2</sub> :
  * >   e<sub>3</sub></i> where <i>e<sub>1</sub></i>, <i>e<sub>2</sub></i> and
  * >   <i>e<sub>3</sub></i> are constant expressions, and <i>e<sub>1</sub></i>
  * >   evaluates to a boolean value.
  * >   </span>
  *
+ * However, this comment is now at least a little bit out of sync with the spec.
+ *
  * The values returned by instances of this class are therefore `null` and
  * instances of the classes `Boolean`, `BigInteger`, `Double`, `String`, and
  * `DartObject`.
  *
  * In addition, this class defines several values that can be returned to
  * indicate various conditions encountered during evaluation. These are
  * documented with the static fields that define those values.
  */
 class ConstantEvaluator extends GeneralizingAstVisitor<Object> {
   /**
@@ skipping 104 matching lines @@
         if (leftOperand is num && rightOperand is num) {
           return leftOperand - rightOperand;
         }
       } else if (node.operator.type == TokenType.PERCENT) {
         // numeric or {@code null}
         if (leftOperand is num && rightOperand is num) {
           return leftOperand.remainder(rightOperand);
         }
       } else if (node.operator.type == TokenType.PLUS) {
         // numeric or {@code null}
-        if (leftOperand is num && rightOperand is num) {
+        if ((leftOperand is num && rightOperand is num) ||
+            (leftOperand is String && rightOperand is String)) {
           return leftOperand + rightOperand;
         }
       } else if (node.operator.type == TokenType.STAR) {
         // numeric or {@code null}
         if (leftOperand is num && rightOperand is num) {
           return leftOperand * rightOperand;
         }
       } else if (node.operator.type == TokenType.SLASH) {
         // numeric or {@code null}
         if (leftOperand is num && rightOperand is num) {
@@ skipping 6834 matching lines @@
     if (node.star != null) {
       sink.write("yield* ");
     } else {
       sink.write("yield ");
     }
     safelyVisitNode(node.expression);
     sink.write(";");
     return null;
   }
 }