Fix tests in language suite: eliminate String +

tests/language/super_field_test.dart

Index: tests/language/super_field_test.dart
===================================================================
--- tests/language/super_field_test.dart	(revision 7724)
+++ tests/language/super_field_test.dart	(working copy)
@@ -18,7 +18,7 @@
 class B extends A {
   B() : super() {}
   String greeting() {
-    return "Hola " + super.greeting();
+    return "Hola ".concat(super.greeting());
   }
 }
 
@@ -26,10 +26,10 @@
 class C extends B {
   C() : super() {}
   String greeting() {
-    return "Servus " + super.greeting();
+    return "Servus ".concat(super.greeting());
   }
   String get city() {
-    return "Basel " + super.city;
+    return "Basel ".concat(super.city);
   }
 }