- Fix use of String operator + in language and corelib tests.

tests/language/src/CallThroughGetterTest.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.
 
 // Tests that we can call functions through getters.
 
 final TOP_LEVEL_CONST = 1;
 final TOP_LEVEL_CONST_REF = TOP_LEVEL_CONST;
 final TOP_LEVEL_NULL = null;
 
@@ skipping 93 matching lines @@
     b = new B();
     Expect.equals("yzxgf", b.g3(b.y, b.z, b.x));
     b = new B();
     Expect.equals("gyzxf", (b.g3)(b.y, b.z, b.x));
   }
 
   static void expectThrowsNotClosure(fn) {
     var exception = catchException(fn);
     if (!(exception is ObjectNotClosureException)) {
       Expect.fail("Wrong exception.  Expected: ObjectNotClosureException"
-          + " got: ${exception}");
+          " got: ${exception}");
     }
   }
 
   static catchException(fn) {
     bool caught = false;
     var result = null;
     try {
       fn();
       Expect.equals(true, false);  // Shouldn't reach this.
     } catch (var e) {
@@ skipping 12 matching lines @@
   A() { }
   var field;
   get getter() { return field; }
   method() { return field; }
 
 }
 
 
 class B {
 
-  B() : _order = "" { }
+  B() : _order = new StringBuffer("") { }
 
   get g0() { _mark('g'); return f0() { return _mark('f'); }; }
   get g1() { _mark('g'); return f1(x) { return _mark('f'); }; }
   get g2() { _mark('g'); return f2(x, y) { return _mark('f'); }; }
   get g3() { _mark('g'); return f3(x, y, z) { return _mark('f'); }; }
 
   get x() { _mark('x'); return 0; }
   get y() { _mark('y'); return 1; }
   get z() { _mark('z'); return 2; }
 
-  _mark(m) { _order += m; return _order; }
-  String _order;
+  _mark(m) { _order.add(m); return _order.toString(); }
+  StringBuffer _order;
 
 }
 
 main() {
   CallThroughGetterTest.testMain();
 }