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..f18f2272b1ac96bf434e8def1f6338eae32daf82
--- /dev/null
+++ b/sdk/lib/_collection_dev/print.dart
@@ -0,0 +1,18 @@
+// 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.

[Lasse Reichstein Nielsen, 2013/10/15 14:12:02]

And it is updated to always match [Zone.current] ?

[floitsch, 2013/10/15 15:56:20]

No need. The function calls Zone.current.print(line).

+ *
+ * Once the function is set the core `print` function calls this closure instead
+ * of [printToConsole].
+ *
+ * This makes it more efficient for dart2js to tree-shake tiny hello-world
+ * programs.

[Lasse Reichstein Nielsen, 2013/10/15 14:12:02]

Too technical.
Just say that it decouples Zones from the core library.

[floitsch, 2013/10/15 15:56:20]

Done.

+ */
+Function printToZone = null;
+
+external void printToConsole(String line);