Avoid patching in dart2dart.

pkg/compiler/lib/src/dart_backend/backend_ast_to_frontend_ast.dart

Index: pkg/compiler/lib/src/dart_backend/backend_ast_to_frontend_ast.dart
diff --git a/pkg/compiler/lib/src/dart_backend/backend_ast_to_frontend_ast.dart b/pkg/compiler/lib/src/dart_backend/backend_ast_to_frontend_ast.dart
index d314f54ebf8f700b5abfa9ae705b4c2863a393f7..96dbcbc5b12a6c691be27ca4d4215f3c7d3914d8 100644
--- a/pkg/compiler/lib/src/dart_backend/backend_ast_to_frontend_ast.dart
+++ b/pkg/compiler/lib/src/dart_backend/backend_ast_to_frontend_ast.dart
@@ -933,13 +933,17 @@ class TreePrinter {
     return new tree.Modifiers(blankList());
   }
 
-  tree.Modifiers makeModifiers({bool isStatic: false,
+  tree.Modifiers makeModifiers({bool isExternal: false,
+                                bool isStatic: false,
                                 bool isAbstract: false,
                                 bool isFactory: false,
                                 bool isConst: false,
                                 bool isFinal: false,
                                 bool isVar: false}) {
     List<tree.Node> nodes = [];
+    if (isExternal) {
+      nodes.add(makeIdentifier('external'));
+    }
     if (isStatic) {
       nodes.add(makeIdentifier('static'));
     }
@@ -971,7 +975,8 @@ class TreePrinter {
 
   tree.Modifiers makeFunctionModifiers(FunctionExpression exp) {
     if (exp.element == null) return makeEmptyModifiers();
-    return makeModifiers(isStatic: exp.element.isStatic,
+    return makeModifiers(isExternal: exp.element.isExternal,
+                         isStatic: exp.element.isStatic,
                          isFactory: exp.element.isFactoryConstructor);
   }