Add image atlas and nine-slice support

test/runtime/models/runtime_node_test.dart

@@ -10,6 +10,14 @@ void main() {
         'type': 'button',
         'parent': 'top_bar',
         'asset': 'dice_normal',
+        'sourceX': 4,
+        'sourceY': 5,
+        'sourceWidth': 64,
+        'sourceHeight': 32,
+        'sliceLeft': 6,
+        'sliceTop': 7,
+        'sliceRight': 8,
+        'sliceBottom': 9,
         'pressedAsset': 'dice_pressed',
         'disabledAsset': 'dice_disabled',
         'animation': 'idle',
@@ -72,6 +80,14 @@ void main() {
       expect(node.type, 'button');
       expect(node.parent, 'top_bar');
       expect(node.asset, 'dice_normal');
+      expect(node.sourceX, 4);
+      expect(node.sourceY, 5);
+      expect(node.sourceWidth, 64);
+      expect(node.sourceHeight, 32);
+      expect(node.sliceLeft, 6);
+      expect(node.sliceTop, 7);
+      expect(node.sliceRight, 8);
+      expect(node.sliceBottom, 9);
       expect(node.pressedAsset, 'dice_pressed');
       expect(node.disabledAsset, 'dice_disabled');
       expect(node.animation, 'idle');
@@ -147,6 +163,14 @@ void main() {
       expect(node.textShadowOffsetX, isNull);
       expect(node.textShadowOffsetY, isNull);
       expect(node.textShadowBlur, isNull);
+      expect(node.sourceX, isNull);
+      expect(node.sourceY, isNull);
+      expect(node.sourceWidth, isNull);
+      expect(node.sourceHeight, isNull);
+      expect(node.sliceLeft, isNull);
+      expect(node.sliceTop, isNull);
+      expect(node.sliceRight, isNull);
+      expect(node.sliceBottom, isNull);
       expect(node.scrollbarVisible, isTrue);
       expect(node.paddingLeft, 0);
       expect(node.paddingTop, 0);
@@ -182,6 +206,14 @@ void main() {
         'paddingBottom': 11,
         'contentWidth': 120,
         'contentHeight': 100,
+        'sourceX': 3,
+        'sourceY': 4,
+        'sourceWidth': 40,
+        'sourceHeight': 41,
+        'sliceLeft': 5,
+        'sliceTop': 6,
+        'sliceRight': 7,
+        'sliceBottom': 8,
         'pressedAsset': 'button_pressed',
         'disabledAsset': 'button_disabled',
         'scrollX': 90,
@@ -213,6 +245,14 @@ void main() {
       expect(updated.paddingBottom, 11);
       expect(updated.contentWidth, 120);
       expect(updated.contentHeight, 100);
+      expect(updated.sourceX, 3);
+      expect(updated.sourceY, 4);
+      expect(updated.sourceWidth, 40);
+      expect(updated.sourceHeight, 41);
+      expect(updated.sliceLeft, 5);
+      expect(updated.sliceTop, 6);
+      expect(updated.sliceRight, 7);
+      expect(updated.sliceBottom, 8);
       expect(updated.pressedAsset, 'button_pressed');
       expect(updated.disabledAsset, 'button_disabled');
       expect(updated.scrollX, 68);
@@ -255,6 +295,22 @@ void main() {
         () => RuntimeNode.fromMap({'id': 'a', 'type': 'progress', 'value': 2}),
         throwsFormatException,
       );
+      expect(
+        () => RuntimeNode.fromMap({
+          'id': 'a',
+          'type': 'image',
+          'sourceWidth': 0,
+        }),
+        throwsFormatException,
+      );
+      expect(
+        () => RuntimeNode.fromMap({
+          'id': 'a',
+          'type': 'image',
+          'sliceLeft': -1,
+        }),
+        throwsFormatException,
+      );
       expect(
         () => RuntimeNode.fromMap({
           'id': 'a',