Use closure tracer to identify closures that are not passed to Function.apply

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 "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:collection';
 
@@ skipping 48 matching lines @@
     var receiver;
   }
   class Closure implements Function {}
   class Null {}
   class Dynamic_ {}
   class LinkedHashMap {
     factory LinkedHashMap._empty() => null;
     factory LinkedHashMap._literal(elements) => null;
   }
   class ConstantMap {}
+  class ConstantStringMap {}
   class TypeImpl {}
   S() {}
   throwCyclicInit() {}
   throwExpression(e) {}
   unwrapException(e) {}
   assertHelper(a) {}
   isJsIndexable(a, b) {}
   createRuntimeType(a) {}
   createInvocationMirror(a0, a1, a2, a3, a4, a5) {}
   throwNoSuchMethod(obj, name, arguments, expectedArgumentNames) {}
@@ skipping 499 matching lines @@
     } else {
       sourceFile = compiler.sourceFiles[uri.toString()];
     }
     if (sourceFile != null && begin != null && end != null) {
       print(sourceFile.getLocationMessage(message, begin, end, true, (x) => x));
     } else {
       print(message);
     }
   };
 }