Support + concatenation in constant expressions with strings

pkg/analyzer/lib/src/dart/ast/utilities.dart

Index: pkg/analyzer/lib/src/dart/ast/utilities.dart
diff --git a/pkg/analyzer/lib/src/dart/ast/utilities.dart b/pkg/analyzer/lib/src/dart/ast/utilities.dart
index 3efd68b24894e652a1fd46e77a69644023dec045..8ac2c16df693767f4f30d52b213241b5ed254bae 100644
--- a/pkg/analyzer/lib/src/dart/ast/utilities.dart
+++ b/pkg/analyzer/lib/src/dart/ast/utilities.dart
@@ -2190,16 +2190,21 @@ class AstComparator implements AstVisitor<bool> {
  * >   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> :
@@ -2208,6 +2213,8 @@ class AstComparator implements AstVisitor<bool> {
  * >   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`.
@@ -2332,7 +2339,8 @@ class ConstantEvaluator extends GeneralizingAstVisitor<Object> {
         }
       } 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) {