Implement "go to" block (#238)

* Implement "go to" block

* Add a missing semicolon

My text editor doesn't automatically insert them and I'm not used to using
semicolons so much. :(

* Implement go-to-random

* Clean up the go-to-random code a bit

* Add rounding to _random_ picks

src/blocks/scratch3_motion.js

@@ -18,6 +18,7 @@ Scratch3MotionBlocks.prototype.getPrimitives = function() {
     return {
         'motion_movesteps': this.moveSteps,
         'motion_gotoxy': this.goToXY,
+        'motion_goto': this.goTo,
         'motion_turnright': this.turnRight,
         'motion_turnleft': this.turnLeft,
         'motion_pointindirection': this.pointInDirection,
@@ -48,6 +49,26 @@ Scratch3MotionBlocks.prototype.goToXY = function (args, util) {
     util.target.setXY(x, y);
 };
 
+Scratch3MotionBlocks.prototype.goTo = function (args, util) {
+    var targetX = 0;
+    var targetY = 0;
+    if (args.TO === '_mouse_') {
+        targetX = util.ioQuery('mouse', 'getX');
+        targetY = util.ioQuery('mouse', 'getY');
+    } else if (args.TO === '_random_') {
+        var stageWidth = this.runtime.constructor.STAGE_WIDTH;
+        var stageHeight = this.runtime.constructor.STAGE_HEIGHT;
+        targetX = Math.round(stageWidth * (Math.random() - 0.5));
+        targetY = Math.round(stageHeight * (Math.random() - 0.5));
+    } else {
+        var goToTarget = this.runtime.getSpriteTargetByName(args.TO);
+        if (!goToTarget) return;
+        targetX = goToTarget.x;
+        targetY = goToTarget.y;
+    }
+    util.target.setXY(targetX, targetY);
+};
+
 Scratch3MotionBlocks.prototype.turnRight = function (args, util) {
     var degrees = Cast.toNumber(args.DEGREES);
     util.target.setDirection(util.target.direction + degrees);

src/engine/runtime.js

@@ -69,6 +69,18 @@ function Runtime () {
     this._cloneCounter = 0;
 }
 
+/**
+ * Width of the stage, in pixels.
+ * @const {number}
+ */
+Runtime.STAGE_WIDTH = 480;
+
+/**
+ * Height of the stage, in pixels.
+ * @const {number}
+ */
+Runtime.STAGE_HEIGHT = 360;
+
 /**
  * Event name for glowing a script.
  * @const {string}