dart2js: Make functions that appear to be unreachable throw.

tests/compiler/dart2js/mock_libraries.dart

Index: tests/compiler/dart2js/mock_libraries.dart
diff --git a/tests/compiler/dart2js/mock_libraries.dart b/tests/compiler/dart2js/mock_libraries.dart
index 7fc0bd210315b96b2fd6ab55cfd2edd731801ff9..202ed8073368897cc3d6716b0a8d24814fafbc26 100644
--- a/tests/compiler/dart2js/mock_libraries.dart
+++ b/tests/compiler/dart2js/mock_libraries.dart
@@ -39,7 +39,10 @@ const Map<String, String> DEFAULT_CORE_LIBRARY = const <String, String>{
         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 {}',