dart2js: Make functions that appear to be unreachable throw.

tests/compiler/dart2js/mock_libraries.dart

 // Copyright (c) 2014, 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 for creating mock versions of platform and internal libraries.
 
 library mock_libraries;
 
 String buildLibrarySource(
     Map<String, String> elementMap,
@@ skipping 21 matching lines @@
       class Deprecated extends Object {
         final String expires;
         const Deprecated(this.expires);
       }''',
   'deprecated': 'const Object deprecated = const Deprecated("next release");',
   'double': r'''
       abstract class double extends num {
         static var NAN = 0;
         static parse(s) {}
       }''',
-  'Function': 'class Function {}',
+  'Function': r'''
+      class Function {
+        static apply(Function fn, List positional, [Map named]) => null;
+      }''',
   'identical': 'bool identical(Object a, Object b) { return true; }',
   'int': 'abstract class int extends num { }',
   'Iterable': 'abstract class Iterable {}',
   'LinkedHashMap': r'''
       class LinkedHashMap {
         factory LinkedHashMap._empty() => null;
         factory LinkedHashMap._literal(elements) => null;
         static _makeEmpty() => null;
         static _makeLiteral(elements) => null;
       }''',
@@ skipping 372 matching lines @@
     final _nestedMaps;
 
     const LookupMap(this._entries, [this._nestedMaps = const []])
         : _key = null, _value = null;
 
     const LookupMap.pair(this._key, this._value)
         : _entries = const [], _nestedMaps = const [];
     V operator[](K k) => null;
   }''',
 };