Make print interceptable.

tests/lib/async/intercept_print1_test.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.
+
+import "package:expect/expect.dart";
+import 'dart:async';
+import 'catch_errors.dart';
+
+var events = [];
+
+void printHandler1(Zone self, ZoneDelegate parent, Zone origin, String line) {
+  events.add("print: $line");
+}
+
+bool shouldIntercept = true;
+
+void printHandler2(Zone self, ZoneDelegate parent, Zone origin, String line) {
+  if (shouldIntercept) {
+    events.add("print **: $line");
+  } else {
+    parent.print(origin, line);
+  }
+}
+
+const TEST_SPEC1 = const ZoneSpecification(print: printHandler1);
+const TEST_SPEC2 = const ZoneSpecification(print: printHandler2);
+
+main() {
+  Zone zone1 = Zone.current.fork(specification: TEST_SPEC1);
+  Zone zone2 = zone1.fork(specification: TEST_SPEC2);
+  zone1.run(() {
+    print("1");
+    print(2);
+    print({3: [4]});
+  });
+  zone2.run(() {
+    print("5");
+    shouldIntercept = false;
+    print(6);
+  });
+  Expect.listEquals(
+      ["print: 1", "print: 2", "print: {3: [4]}", "print **: 5", "print: 6"],
+      events);
+}