index.html

<div id="score" class="score"></div>

<canvas id="myCanvas" width="500" height="500"></canvas>

main.dart

// Copyright 2014 David Kopec 
// Created for Dart for Absolute Beginners, an Apress title
// MIT License, see https://github.com/davecom/Dart-for-Absolute-Beginners/blob/master/license.txt

import 'dart:html';
import 'dart:async';  // for Timer
import 'dart:math';  // for Random

const int BOX_WIDTH = 100;
const int BOX_HEIGHT = 50;

int boxX, boxY, score = 0, speed;
CanvasElement myCanvas;

Element get scoreElement => querySelector('#score');

/// Called by a Timer 60 times per second
void update(Timer t) {
  boxX += speed;  //update box's location
  // get a new box when the last one has gone off screen
  if (boxX < (-BOX_WIDTH) || boxX > myCanvas.width){
    newRandomBox();
  }
  draw();
}

/// Draw a background, the box, and the score
void draw() {
  CanvasRenderingContext2D myCanvasContext = myCanvas.context2D;
  //draw the background
  myCanvasContext.setFillColorRgb(255, 255, 255);  // Blue
  myCanvasContext.fillRect(0, 0, 500, 500);  // 0x, 0y, 500 width, 500 height
  //draw the score in black at the top right of the screen
  String scoreText = "Score: $score";
  //myCanvasContext.fillText(scoreText, myCanvas.width-100, 30); // string, x, y
  scoreElement.text = scoreText;
  myCanvasContext.setFillColorRgb(0, 0, 0);  // Black
  //draw the box
  myCanvasContext.fillRect(boxX,boxY,BOX_WIDTH,BOX_HEIGHT);
}

/// Sets up a new box at a random location
void newRandomBox() {
  // if it's 1 it will go right, otherwise left
  Random rand = new Random();
  speed = rand.nextInt(10) + 5;  // random speed 5 to 14 pixels/frame
  boxY = rand.nextInt(myCanvas.height - BOX_HEIGHT);  // random y
  int leftOrRight = rand.nextInt(2);
  if (leftOrRight == 1) {  // going from left to right
    boxX = 0;
  } else {   //going from right to left
    boxX = myCanvas.width - BOX_WIDTH;
    speed = -speed;  // move left not right
  }
}

/// When the canvas is clicked, we need to check if the user hit a box
void clickHappened(MouseEvent me) {
  //check if click was within the box's space
  int clickX = me.offset.x;
  int clickY = me.offset.y;
  if (clickX > boxX && clickX < boxX + BOX_WIDTH 
      && clickY > boxY && clickY < boxY + BOX_HEIGHT) {  // we have a hit
    score++;
    newRandomBox();
  }
}

void main() {
  myCanvas = querySelector("#myCanvas");
  myCanvas.onClick.listen(clickHappened);
  newRandomBox();
  // TODO: We should we requestAnimationFrame here.
  // This Timer will call update() approximately 60 times a second
  new Timer.periodic(const Duration(milliseconds:17), update);
}

style.css

.score {
  
}