Import dom/ from web-platform-tests

third_party/WebKit/LayoutTests/imported/web-platform-tests/dom/constants.js

+function testConstants(objects, constants, msg) {
+  objects.forEach(function(arr) {
+    var o = arr[0], desc = arr[1];
+    test(function() {
+      constants.forEach(function(d) {
+        assert_true(d[0] in o, "Object " + o + " doesn't have " + d[0])
+        assert_equals(o[d[0]], d[1], "Object " + o + " value for " + d[0] + " is wrong")
+      })
+    }, "Constants for " + msg + " on " + desc + ".")
+  })
+}