Don't crash if you call document.createText(null)

sky/tests/raw/null_text_crash.dart

Index: sky/tests/raw/null_text_crash.dart
diff --git a/sky/tests/framework/color_accessors.dart b/sky/tests/raw/null_text_crash.dart
similarity index 54%
copy from sky/tests/framework/color_accessors.dart
copy to sky/tests/raw/null_text_crash.dart
index 93b03c670c0314c04c22c3e1a7b740c923cc803c..441646ab2a8ec4df5ab1a1428dbae563bbae0771 100644
--- a/sky/tests/framework/color_accessors.dart
+++ b/sky/tests/raw/null_text_crash.dart
@@ -2,18 +2,16 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:sky';
+import 'dart:sky' as sky;
+
 import '../resources/third_party/unittest/unittest.dart';
 import '../resources/unit.dart';
 
 void main() {
   initUnit();
 
-  test("color accessors should work", () {
-    Color foo = new Color(0x12345678);
-    expect(foo.alpha, equals(0x12));
-    expect(foo.red, equals(0x34));
-    expect(foo.green, equals(0x56));
-    expect(foo.blue, equals(0x78));
+  test("createText(null) shouldn't crash", () {
+    var doc = new sky.Document();
+    doc.createText(null);
   });
 }