Fixes issues in string interpolation tests

tests/language/src/StringInterpolateTest.dart

 // Copyright (c) 2011, 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.
 // Dart test program testing string interpolation.
 
 
 class WhatchamaCallIt {
   WhatchamaCallIt() { }
 
-  void foo() {
-    return "Hansel and $name";  // Field name is defined in subclass.
+  String foo() {
+    // Test $this and Field name is defined in subclass.
+    return "$this and $name";
   }
 }
 
 class ThingamaBob extends WhatchamaCallIt {
   ThingamaBob(String s) : super(), name = s { }
   String name;
+  toString() => "Hansel";
 }
 
 class StringInterpolateTest {
 
   static final String A = "svin";
   static final String B = "hest";
   static final int N = 1 + 1;
   static String Printers;
   static String AAR_Printers;
 
   static testMain() {
     Printers = "Printers: $A and $B";
     AAR_Printers  = "AAR has $N $Printers.";
-    
+
     var x = 1;
     var s = "eins und \$x macht zwei.";
     print(s);
     Expect.equals(@"eins und $x macht zwei.", s);
 
     s = "eins und $x macht zwei.";
     print(s);
     Expect.equals(@"eins und 1 macht zwei.", s);
 
     print(AAR_Printers);
@@ skipping 10 matching lines @@
 
     var t = new ThingamaBob("Gretel");
     print(t.foo());
     Expect.equals(t.foo(), "Hansel and Gretel");
   }
 }
 
 main() {
   StringInterpolateTest.testMain();
 }