Fix tests in language suite: eliminate String +

tests/language/super_setter_test.dart

Index: tests/language/super_setter_test.dart
===================================================================
--- tests/language/super_setter_test.dart	(revision 7724)
+++ tests/language/super_setter_test.dart	(working copy)
@@ -9,7 +9,7 @@
 
   String get value() { return value_; }
   String set value(String newValue) {
-    value_ = 'Base:' + newValue;
+    value_ = 'Base:$newValue';
   }
 }
 
@@ -18,7 +18,7 @@
   Derived() : super() {}
 
   String set value(String newValue) {
-    super.value = 'Derived:' + newValue;
+    super.value = 'Derived:$newValue';
   }
   String get value() { return super.value; }
 }