Adding supported checks for touch event.

tools/dom/templates/html/impl/impl_TouchEvent.darttemplate

Index: tools/dom/templates/html/impl/impl_TouchEvent.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_TouchEvent.darttemplate b/tools/dom/templates/html/impl/impl_TouchEvent.darttemplate
index ea0bef98c19d041c140b3ea30e98b681f5f9a254..3944652d590734e79b42d34880d6ee64acc9a635 100644
--- a/tools/dom/templates/html/impl/impl_TouchEvent.darttemplate
+++ b/tools/dom/templates/html/impl/impl_TouchEvent.darttemplate
@@ -21,4 +21,24 @@ $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
     return e;
   }
 $!MEMBERS
+
+  /**
+   * Checks if touch events supported on the current platform.
+   *
+   * Note that touch events are only supported if the user is using a touch
+   * device.
+   */
+  static bool get supported {
+$if DART2JS
+    if (JS('bool', '"ontouchstart" in window')) {
+      return Event._isTypeSupported('TouchEvent');
+    }
+    return false;
+$else
+    // TODO: add equivalent 'ontouchstart' check for Dartium.

[Emily Fortuna, 2013/01/29 18:19:21]

TOOD(blois): ...

+    // Basically, this is a fairly common check and it'd be great if it did not
+    // throw exceptions.
+    return Event._isTypeSupported('TouchEvent');
+$endif
+  }
 }