Fix some typescript issues in //y2020/www/...

These were revealed by the typescript compiler upgrade.

Change-Id: I548ac2601c9ce830beda14b51272e04ef07bcd00
diff --git a/y2020/www/field_handler.ts b/y2020/www/field_handler.ts
index 5053ef6..ffb4aee 100644
--- a/y2020/www/field_handler.ts
+++ b/y2020/www/field_handler.ts
@@ -84,7 +84,7 @@
 export class FieldHandler {
   private canvas = document.createElement('canvas');
   private imageMatchResult: frc971.vision.sift.ImageMatchResult|null = null;
-  private drivetrainStatus: DrivetrianStatus|null = null;
+  private drivetrainStatus: DrivetrainStatus|null = null;
 
   constructor(private readonly connection: Connection) {
     document.body.appendChild(this.canvas);
@@ -238,7 +238,7 @@
     this.drawField();
     //draw cameras
     if (this.imageMatchResult) {
-      for (const i = 0; i < this.imageMatchResult.cameraPosesLength(); i++) {
+      for (let i = 0; i < this.imageMatchResult.cameraPosesLength(); i++) {
         const pose = this.imageMatchResult.cameraPoses(i);
         const mat = pose.fieldToCamera();
         const x = mat.data(3);