Make print interceptable.

sdk/lib/_collection_dev/print.dart

Index: sdk/lib/_collection_dev/print.dart
diff --git a/sdk/lib/_collection_dev/print.dart b/sdk/lib/_collection_dev/print.dart
new file mode 100644
index 0000000000000000000000000000000000000000..0c7182e0ad3d613eb0f2a3c0838a7d9b06080491
--- /dev/null
+++ b/sdk/lib/_collection_dev/print.dart
@@ -0,0 +1,17 @@
+// 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 dart._collection.dev;
+
+/**
+ * This function is set by the first allocation of a Zone.
+ *
+ * Once the function is set the core `print` function calls this closure instead
+ * of [printToConsole].
+ *
+ * This decouples the core library from the async library.
+ */
+Function printToZone = null;
+
+external void printToConsole(String line);