small updates.

Runtime/flow/opacity_layer.cs

 
         public override void preroll(PrerollContext context, Matrix3 matrix) {
             var childMatrix = new Matrix3(matrix);
-            childMatrix.postTranslate(this._offset.dx, this._offset.dy); // TOOD: pre or post? https://github.com/flutter/engine/pull/7945
+            childMatrix.preTranslate(this._offset.dx, this._offset.dy); // TOOD: pre or post? https://github.com/flutter/engine/pull/7945
             
             base.preroll(context, childMatrix);
             

Runtime/flow/picture_layer.cs

         public override void preroll(PrerollContext context, Matrix3 matrix) {
             if (context.rasterCache != null) {
                 Matrix3 ctm = new Matrix3(matrix);
-                ctm.postTranslate(this._offset.dx, this._offset.dy); // TOOD: pre or post? https://github.com/flutter/engine/pull/7945
+                ctm.preTranslate(this._offset.dx, this._offset.dy); // TOOD: pre or post? https://github.com/flutter/engine/pull/7945
                 ctm[2] = ctm[2].alignToPixel(context.devicePixelRatio);
                 ctm[5] = ctm[5].alignToPixel(context.devicePixelRatio);
 

Runtime/flow/raster_cache.cs

             D.assert(matrix != null);
             this.picture = picture;
             this.matrix = new Matrix3(matrix);
-            var x = this.matrix[2] * devicePixelRatio;
-            var y = this.matrix[5] * devicePixelRatio;
-            this.matrix[2] = (x - (int) x) / devicePixelRatio; // x
-            this.matrix[5] = (y - (int) y) / devicePixelRatio; // y
-            D.assert(this.matrix[2] == 0);
-            D.assert(this.matrix[5] == 0);
+            D.assert(() => {
+                var x = this.matrix[2] * devicePixelRatio;
+                var y = this.matrix[5] * devicePixelRatio;
+                this.matrix[2] = (x - (int) x) / devicePixelRatio; // x
+                this.matrix[5] = (y - (int) y) / devicePixelRatio; // y
+                
+                D.assert(Mathf.Abs(this.matrix[2]) <= 1e-5);
+                D.assert(Mathf.Abs(this.matrix[5]) <= 1e-5);
+                return true;
+            });
+            
+            this.matrix[2] = 0.0f;
+            this.matrix[5] = 0.0f;            
             this.devicePixelRatio = devicePixelRatio;
         }