Support + concatenation in constant expressions with strings

pkg/analyzer/test/src/dart/ast/utilities_test.dart

Index: pkg/analyzer/test/src/dart/ast/utilities_test.dart
diff --git a/pkg/analyzer/test/src/dart/ast/utilities_test.dart b/pkg/analyzer/test/src/dart/ast/utilities_test.dart
index 368c30700e55876299c1f7c669e4f566cfc9aa2b..c0ae90a28db0444b99c11d17008e15a14889f9d6 100644
--- a/pkg/analyzer/test/src/dart/ast/utilities_test.dart
+++ b/pkg/analyzer/test/src/dart/ast/utilities_test.dart
@@ -196,6 +196,11 @@ class ConstantEvaluatorTest extends ParserTestCase {
     expect(value, true);
   }
 
+  void test_binary_plus_string() {
+    Object value = _getConstantValue("'hello ' + 'world'");
+    expect(value, 'hello world');
+  }
+
   void test_binary_plus_double() {
     Object value = _getConstantValue("2.3 + 3.2");
     expect(value, 2.3 + 3.2);
@@ -206,6 +211,11 @@ class ConstantEvaluatorTest extends ParserTestCase {
     expect(value, 5);
   }
 
+  void test_binary_plus_string_mixed() {

[scheglov, 2016/12/22 23:22:54]

Maybe worth to test when both case for num/string combination:
test_binary_plus_int_string and test_binary_plus_string_int?
And maybe also double?

[mfairhurst, 2016/12/23 00:04:41]

Acknowledged.

+    Object value = _getConstantValue("5 + 'world'");
+    expect(value, ConstantEvaluator.NOT_A_CONSTANT);
+  }
+
   void test_binary_remainder_double() {
     Object value = _getConstantValue("3.2 % 2.3");
     expect(value, 3.2 % 2.3);