Avoid patching in dart2dart.

pkg/compiler/lib/src/cps_ir/cps_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 '../constants/expressions.dart';
 import '../constants/values.dart' show PrimitiveConstantValue;
 import '../dart_backend/dart_backend.dart' show DartBackend;
 import '../dart_types.dart';
@@ skipping 522 matching lines @@
     _current = null;
   }
 
   /// Create a [ir.FunctionDefinition] for [element] using [_root] as the body.
   ///
   /// Parameters must be created before the construction of the body using
   /// [createParameter].
   ir.FunctionDefinition buildFunctionDefinition(
       FunctionElement element,
       List<ConstantExpression> defaults) {
-    if (!element.isAbstract) {
-      _ensureReturn();
-      return new ir.FunctionDefinition(
-          element, state.returnContinuation, _parameters, _root,
-          state.localConstants, defaults);
-    } else {
+    if (element.isAbstract || element.isExternal) {
       assert(invariant(element, _root == null,
           message: "Non-empty body for abstract method $element: $_root"));
       assert(invariant(element, state.localConstants.isEmpty,
           message: "Local constants for abstract method $element: "
                    "${state.localConstants}"));
       return new ir.FunctionDefinition.abstract(
                 element, _parameters, defaults);
+    } else {
+      _ensureReturn();
+      return new ir.FunctionDefinition(
+          element, state.returnContinuation, _parameters, _root,
+          state.localConstants, defaults);
     }
   }
 
   /// Create a super invocation where the method name and the argument structure
   /// are defined by [selector] and the argument values are defined by
   /// [arguments].
   ir.Primitive buildSuperInvocation(Selector selector,
                                     List<ir.Primitive> arguments) {
     return _buildInvokeSuper(selector, arguments);
   }
@@ skipping 888 matching lines @@
     index = 0;
     for (int i = 0; i < environment.length; ++i) {
       if (common[i] == null) {
         environment.index2value[i] = parameters[index++];
       }
     }
 
     return join;
   }
 }