Fix incompatibility of PrintStream.flush

...which should not throw IOException.

Change-Id: If216fc6054a14db4cd17549a0898dde3f8598b15
diff --git a/user/super/com/google/gwt/emul/java/io/PrintStream.java b/user/super/com/google/gwt/emul/java/io/PrintStream.java
index 47163e5..fb2739a 100644
--- a/user/super/com/google/gwt/emul/java/io/PrintStream.java
+++ b/user/super/com/google/gwt/emul/java/io/PrintStream.java
@@ -81,4 +81,6 @@
   public void println(String s) {
   }
 
+  public void flush() {
+  }
 }