[Chromoting] Decouple DesktopView and TouchInputHandler

remoting/android/java/src/org/chromium/chromoting/Preconditions.java

Index: remoting/android/java/src/org/chromium/chromoting/Preconditions.java
diff --git a/remoting/android/java/src/org/chromium/chromoting/Preconditions.java b/remoting/android/java/src/org/chromium/chromoting/Preconditions.java
index 5df5b781704c74fafabf88aa308c686246b8ce8b..963153754043460bce79b2cfa4b5f8d9b5b52bec 100644
--- a/remoting/android/java/src/org/chromium/chromoting/Preconditions.java
+++ b/remoting/android/java/src/org/chromium/chromoting/Preconditions.java
@@ -25,7 +25,7 @@ public final class Preconditions {
     }
 
     /**
-     * Checks whether input |ref| is not a null reference, and return its value. Throws
+     * Checks whether input |ref| is not a null reference, and returns its value. Throws
      * {@link NullPointerException} if |ref| is null.
      */
     public static final <T> T notNull(T ref) {
@@ -34,4 +34,15 @@ public final class Preconditions {
         }
         return ref;
     }
+
+    /**
+     * Checks whether input |ref| is a null reference, and returns its value. Throws
+     * {@link IllegalArgumentException} if |ref| is not null.
+     */
+    public static final <T> T isNull(T ref) {
+        if (ref != null) {
+            throw new IllegalArgumentException();
+        }
+        return ref;
+    }
 }