Make print interceptable.

runtime/lib/print_patch.dart

 // Copyright (c) 2012, 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.
 
 typedef void _PrintClosure(Object obj);
 
-patch void print(Object obj) {
-  _printClosure(obj.toString());
+patch void printToConsole(String line) {
+  _printClosure(line);
 }
 
 void _unsupportedPrint(Object obj) {
   throw new UnsupportedError("'print' is not supported");
 }
 
 // _printClosure can be overwritten by the embedder to supply a different
 // print implementation.
 _PrintClosure _printClosure = _unsupportedPrint;