Implement dynamic function checks.

dart/sdk/lib/_internal/compiler/implementation/ir/ir_builder.dart

 // Copyright (c) 2013, 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 dart2js.ir_builder;
 
 import 'ir_nodes.dart';
 import '../elements/elements.dart';
 import '../dart2jslib.dart';
 import '../source_file.dart';
@@ skipping 98 matching lines @@
     if (backend.isInterceptedMethod(element)) return false;
 
     // TODO(lry): support native functions (also in [visitReturn]).
     if (function.isNative()) return false;
 
     return true;
   }
 
   void unlinkTreeAndToken(element) {
     element.beginToken.next = null;
-    element.cachedNode = null;
+    // TODO(lry): Mark [element] so that parseNode will fail an assertion.
+    // element.cachedNode = null;
   }
 
   SourceFile elementSourceFile(Element element) {
     if (element is FunctionElement) {
       FunctionElement functionElement = element;
       if (functionElement.patch != null) element = functionElement.patch;
     }
     return element.getCompilationUnit().script.file;
   }
 }
@@ skipping 196 matching lines @@
 
   void exitBlock() {
     blockBuilders.removeLast();
   }
 }
 
 class BlockBuilder {
   List<IrNode> statements = <IrNode>[];
   bool hasReturn = false;
 }