Enable antialiasing on Paint by default.

The app looks so much better with this on.

R=jackson@google.com, abarth@chromium.org

Review URL: https://codereview.chromium.org/1172413002.

engine/core/painting/Paint.cpp

@@ -14,6 +14,7 @@ namespace blink {
 
 Paint::Paint()
 {
+    setIsAntiAlias(true);
 }
 
 Paint::~Paint()

sdk/lib/framework/components2/checkbox.dart

@@ -43,7 +43,6 @@ class Checkbox extends ButtonBase {
           callback: (sky.Canvas canvas) {
 
             sky.Paint paint = new sky.Paint()..color = color
-                                             ..isAntiAlias = true
                                              ..strokeWidth = 2.0;
 
             // Draw the outer rrect