Add a MaskFilter interface to dart:sky to handle blur.

sky/engine/core/painting/MaskFilter.dart

Index: sky/engine/core/painting/MaskFilter.dart
diff --git a/sky/engine/core/painting/MaskFilter.dart b/sky/engine/core/painting/MaskFilter.dart
new file mode 100644
index 0000000000000000000000000000000000000000..c7014509bac72f29d91c4b5570f41c2fc84f769b
--- /dev/null
+++ b/sky/engine/core/painting/MaskFilter.dart
@@ -0,0 +1,27 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Extends the generated _MaskFilter interface via the CustomDart attribute.
+

[eseidel, 2015/06/02 20:49:45]

Do we need "part dart:sky" lines at the top of these?

[Matt Perry, 2015/06/02 21:03:42]

Not for this one - CustomDart files are appended to the end of the generated
code.

This distinction is confusing... I need to find a way to unify the different
ways of doing custom dart code.

+/// Blur styles. These mirror SkBlurStyle and must be kept in sync.
+enum BlurStyle {
+  normal,  /// Fuzzy inside and outside.

[eseidel, 2015/06/02 20:49:45]

Is this the correct Dart enum naming style?

[Matt Perry, 2015/06/02 21:03:42]

According to https://www.dartlang.org/articles/style-guide/#names . But if we
want a different enum style, I'm happy to use that.

+  solid,  /// Solid inside, fuzzy outside.
+  outer,  /// Nothing inside, fuzzy outside.
+  inner,  /// Fuzzy inside, nothing outside.
+}
+
+class MaskFilter extends _MaskFilter {
+  MaskFilter.Blur(BlurStyle style, double sigma,
+                  {bool ignoreTransform: false, bool highQuality: false})
+      : super(style.index, sigma, _makeBlurFlags(ignoreTransform, highQuality));
+
+  // Convert constructor parameters to the SkBlurMaskFilter::BlurFlags type.
+  static int _makeBlurFlags(bool ignoreTransform, bool highQuality) {
+    int flags = 0;
+    if (ignoreTransform) flags |= 0x01;
+    if (highQuality) flags |= 0x02;
+    return flags;
+  }
+}