Support + concatenation in constant expressions with strings

pkg/analyzer/test/src/dart/ast/utilities_test.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.test.src.dart.ast.utilities_test;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_ast_factory.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 178 matching lines @@
   void test_binary_notEqual_invalidRight() {
     Object value = _getConstantValue("2 != a");
     expect(value, ConstantEvaluator.NOT_A_CONSTANT);
   }
 
   void test_binary_notEqual_string() {
     Object value = _getConstantValue("'a' != 'b'");
     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);
   }
 
   void test_binary_plus_integer() {
     Object value = _getConstantValue("2 + 3");
     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);
   }
 
   void test_binary_remainder_integer() {
     Object value = _getConstantValue("8 % 3");
     expect(value, 2);
   }
 
@@ skipping 5549 matching lines @@
   /**
    * Assert that a `ToSourceVisitor` will produce the [expectedSource] when
    * visiting the given [node].
    */
   void _assertSource(String expectedSource, AstNode node) {
     PrintStringWriter writer = new PrintStringWriter();
     node.accept(new ToSourceVisitor(writer));
     expect(writer.toString(), expectedSource);
   }
 }