Fix braino that lead to dead code that contained a typo. Also add a regression test.

tests/compiler/dart2js/mock_compiler.dart

 // Copyright (c) 2012, 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.
 
 library mock_compiler;
 
 import 'dart:collection';
 import 'dart:uri';
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart' as api;
@@ skipping 14 matching lines @@
 
 class WarningMessage {
   Node node;
   Message message;
   WarningMessage(this.node, this.message);
 
   toString() => message.toString();
 }
 
 const String DEFAULT_HELPERLIB = r'''
-  lt() {} add(var a, var b) {} sub() {} mul() {} div() {} tdiv() {} mod() {}
-  neg() {} shl() {} shr() {} eq() {} le() {} gt() {} ge() {}
-  or() {} and() {} not() {} eqNull(a) {} eqq() {}
-  ltB() {} leB() {} eqB() {} gtB() {} geB() {} eqNullB(a) {}
   $throw(x) { return x; }
   iae(x) { throw x; } ioore(x) { throw x; }
   guard$array(x) { return x; }
   guard$num(x) { return x; }
   guard$string(x) { return x; }
   guard$stringOrArray(x) { return x; }
-  index(a, index) {}
-  indexSet(a, index, value) {}
   makeLiteralMap(List keyValuePairs) {}
   setRuntimeTypeInfo(a, b) {}
   getRuntimeTypeInfo(a) {}
   stringTypeCheck(x) {}
   boolConversionCheck(x) {}
   abstract class JavaScriptIndexingBehavior {}
   class JSInvocationMirror {}
   S() {}
   assertHelper(a){}
   throwNoSuchMethod(obj, name, arguments, expectedArgumentNames) {}''';
@@ skipping 44 matching lines @@
 
 const String DEFAULT_CORELIB = r'''
   print(var obj) {}
   abstract class num {}
   abstract class int extends num { }
   abstract class double extends num { }
   class bool {}
   class String {}
   class Object {
     operator ==(other) {}
+    String toString() {}
   }
   class Type {}
   class Function {}
   class List<E> {}
   abstract class Map<K,V> {}
   class Closure {}
   class Null {}
   class Dynamic_ {}
   bool identical(Object a, Object b) {}''';
 
@@ skipping 209 matching lines @@
   operator []=(Node node, Element element) {
     map[node] = element;
   }
 
   operator [](Node node) => map[node];
 
   void remove(Node node) {
     map.remove(node);
   }
 }