kernel -> ssa: add tests for other simple operators

R=sigmund@google.com

Review URL: https://codereview.chromium.org/2299453004 .

tests/compiler/dart2js/kernel/binary_operators_test.dart

@@ -11,5 +11,17 @@ main() {
     test('plus on ints', () {
       return check('main() { return 1 + 2; }');
     });
+    test('plus on strings', () {
+      return check('main() { return "a" + "b"; }');
+    });
+    test('plus on non-constants', () {
+      String code = '''
+        foo() => 1;
+        main() => foo() + foo();''';
+      return check(code);
+    });
+    test('other arithmetic operators', () {
+      return check('main() { return 1 + 2 * 3 - 4 / 5 % 6; }');
+    });
   });
 }