Downgrade typescript to 2.8.3

This is to exactly match the runtime in the prototype for easier
debugging. We will upgrade once the runtime is functional being tested
properly.

package.json

@@ -9,7 +9,7 @@
  "tslint": "^5.10.0",
  "tslint-eslint-rules": "^5.3.1",
  "tslint-no-circular-imports": "^0.5.0",
- "typescript": "^2.9.1"
+ "typescript": "2.8.3"
  },
  "browserslist": [
  "chrome 69"

yarn.lock

@@ -3513,9 +3513,9 @@ typedarray@^0.0.6:
  version "0.0.6"
  resolved "https://registry.yarnpkg.com/typedarray/-/typedarray-0.0.6.tgz#867ac74e3864187b1d3d47d996a78ec5c8830777"
 
-typescript@^2.9.1:
- version "2.9.1"
- resolved "https://registry.yarnpkg.com/typescript/-/typescript-2.9.1.tgz#fdb19d2c67a15d11995fd15640e373e09ab09961"
+typescript@2.8.3:
+ version "2.8.3"
+ resolved "https://registry.yarnpkg.com/typescript/-/typescript-2.8.3.tgz#5d817f9b6f31bb871835f4edf0089f21abe6c170"
 
 uglify-es@^3.2.1, uglify-es@^3.3.9:
  version "3.3.9"