added "checked box" symbol

snap.html

@@ -5,7 +5,7 @@
         <title>Snap! Build Your Own Blocks 6.0.0 - beta -</title>
         <link rel="shortcut icon" href="src/favicon.ico">
         <script type="text/javascript" src="src/morphic.js?version=2020-05-19"></script>
-        <script type="text/javascript" src="src/symbols.js?version=2020-05-27"></script>
+        <script type="text/javascript" src="src/symbols.js?version=2020-05-28"></script>
         <script type="text/javascript" src="src/widgets.js?version=2020-05-06"></script>
         <script type="text/javascript" src="src/blocks.js?version=2020-05-25"></script>
         <script type="text/javascript" src="src/threads.js?version=2020-05-27"></script>

src/symbols.js

@@ -41,7 +41,7 @@
 
 // Global stuff ////////////////////////////////////////////////////////
 
-modules.symbols = '2020-May-27';
+modules.symbols = '2020-May-28';
 
 var SymbolMorph;
 
@@ -92,6 +92,7 @@ SymbolMorph.prototype.names = [
     'flash',
     'brush',
     'rectangle',
+    'checkedBox',
     'rectangleSolid',
     'circle',
     'circleSolid',
@@ -303,6 +304,9 @@ SymbolMorph.prototype.renderShape = function (ctx, aColor) {
     case 'rectangle':
         this.renderSymbolRectangle(ctx, aColor);
         break;
+    case 'checkedBox':
+        this.renderSymbolCheckedBox(ctx, aColor);
+        break;
     case 'rectangleSolid':
         this.renderSymbolRectangleSolid(ctx, aColor);
         break;
@@ -1045,6 +1049,23 @@ SymbolMorph.prototype.renderSymbolRectangle = function (ctx, color) {
     ctx.stroke();
 };
 
+SymbolMorph.prototype.renderSymbolCheckedBox = function (ctx, color) {
+    // draw a rectangle with a check mark
+    var w = this.symbolWidth(),
+        h = this.size,
+        l = Math.max(w / 20, 0.5);
+
+    this.renderSymbolRectangle(ctx, color);
+    ctx.fillStyle = color.toString();
+    ctx.beginPath();
+    ctx.moveTo(l, h * 0.4);
+    ctx.lineTo(w * 0.5, h - l);
+    ctx.lineTo(w - l, l);
+    ctx.lineTo(w * 0.5, h * 0.65);
+    ctx.closePath();
+    ctx.fill();
+};
+
 SymbolMorph.prototype.renderSymbolRectangleSolid = function (ctx, color) {
     // draw a solid rectangle
     var w = this.symbolWidth(),