Add Blockly.BlockSvg.START_HAT option.

core/block_svg.js

@@ -851,6 +851,28 @@ Blockly.BlockSvg.NOTCH_WIDTH = 30;
  * @const
  */
 Blockly.BlockSvg.CORNER_RADIUS = 8;
+/**
+ * Do blocks with no previous or output connections have a 'hat' on top?
+ * @const
+ */
+Blockly.BlockSvg.START_HAT = false;
+/**
+ * Path of the top hat's curve.
+ * @const
+ */
+Blockly.BlockSvg.START_HAT_PATH = 'c 30,-15 70,-15 100,0';
+/**
+ * Path of the top hat's curve's highlight in LTR.
+ * @const
+ */
+Blockly.BlockSvg.START_HAT_HIGHLIGHT_LTR =
+    'c 17.8,-9.2 45.3,-14.9 75,-8.7 M 100.5,0.5';
+/**
+ * Path of the top hat's curve's highlight in RTL.
+ * @const
+ */
+Blockly.BlockSvg.START_HAT_HIGHLIGHT_RTL =
+    'm 25,-8.7 c 29.7,-6.2 57.2,-0.5 75,8.7';
 /**
  * Distance from shape edge to intersect with a curved corner at 45 degrees.
  * Applies to highlighting on around the inside of a curve.
@@ -1704,6 +1726,7 @@ Blockly.BlockSvg.prototype.renderCompute_ = function(iconWidth) {
  * @private
  */
 Blockly.BlockSvg.prototype.renderDraw_ = function(iconWidth, inputRows) {
+  this.startHat_ = false;
   // Should the top and bottom left corners be rounded or square?
   if (this.outputConnection) {
     this.squareTopLeftCorner_ = true;
@@ -1717,6 +1740,11 @@ Blockly.BlockSvg.prototype.renderDraw_ = function(iconWidth, inputRows) {
       if (prevBlock && prevBlock.getNextBlock() == this) {
         this.squareTopLeftCorner_ = true;
        }
+    } else if (Blockly.BlockSvg.START_HAT) {
+      // No output or previous connection.
+      this.squareTopLeftCorner_ = true;
+      this.startHat_ = true;
+      inputRows.rightEdge = Math.max(inputRows.rightEdge, 100);
     }
     var nextBlock = this.getNextBlock();
     if (nextBlock) {
@@ -1771,6 +1799,12 @@ Blockly.BlockSvg.prototype.renderDrawTop_ =
   if (this.squareTopLeftCorner_) {
     steps.push('m 0,0');
     highlightSteps.push('m 0.5,0.5');
+    if (this.startHat_) {
+      steps.push(Blockly.BlockSvg.START_HAT_PATH);
+      highlightSteps.push(this.RTL ?
+          Blockly.BlockSvg.START_HAT_HIGHLIGHT_RTL :
+          Blockly.BlockSvg.START_HAT_HIGHLIGHT_LTR);
+    }
   } else {
     steps.push(Blockly.BlockSvg.TOP_LEFT_CORNER_START);
     highlightSteps.push(this.RTL ?