Avoid patching in dart2dart.

pkg/compiler/lib/src/cps_ir/cps_ir_builder_visitor.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.
 
 part of dart2js.ir_builder;
 
 /**
  * This task iterates through all resolved elements and builds [ir.Node]s. The
  * nodes are stored in the [nodes] map and accessible through [hasIr] and
  * [getIr].
@@ skipping 123 matching lines @@
    * returns `null`.
    */
   ir.FunctionDefinition buildFunction(FunctionElement functionElement) {
     return nullIfGiveup(() => buildFunctionInternal(functionElement));
   }
 
   ir.FunctionDefinition buildFunctionInternal(FunctionElement element) {
     assert(invariant(element, element.isImplementation));
     ast.FunctionExpression function = element.node;
     assert(function != null);
-    assert(!function.modifiers.isExternal);
     assert(elements[function] != null);
 
     DetectClosureVariables closureLocals = new DetectClosureVariables(elements);
     closureLocals.visit(function);
 
     return withBuilder(
         new IrBuilder(compiler.backend.constantSystem,
                       element, closureLocals.usedFromClosure),
         () {
       FunctionSignature signature = element.functionSignature;
@@ skipping 721 matching lines @@
     node.visitChildren(this);
   }
 
   visitFunctionExpression(ast.FunctionExpression node) {
     FunctionElement oldFunction = currentFunction;
     currentFunction = elements[node];
     visit(node.body);
     currentFunction = oldFunction;
   }
 }