Enable strict null checks and restrict DOM access in common

Part of xtermjs#1507
Part of xtermjs#1319

package.json

@@ -51,6 +51,7 @@
     "start": "node demo/start",
     "start-zmodem": "node demo/zmodem/app",
     "lint": "tslint 'src/**/*.ts' './demo/**/*.ts'",
+    "pretest": "npm run layering",
     "test": "npm-run-all mocha lint",
     "test-debug": "node --inspect-brk node_modules/.bin/gulp test",
     "test-suite": "gulp mocha-suite --test",
@@ -64,6 +65,7 @@
     "coveralls": "nyc report --reporter=text-lcov | coveralls",
     "webpack": "gulp webpack",
     "watch": "concurrently --kill-others-on-fail --names \"lib,css\" \"tsc -w\" \"gulp watch-css\"",
-    "watch-addons": "concurrently --kill-others-on-fail --names \"attach,fit,fullscreen,search,terminado,webLinks,winptyCompat,zmodem\" \"tsc -w -p ./src/addons/attach\" \"tsc -w -p ./src/addons/fit\" \"tsc -w -p ./src/addons/fullscreen\" \"tsc -w -p ./src/addons/search\" \"tsc -w -p ./src/addons/terminado\" \"tsc -w -p ./src/addons/webLinks\" \"tsc -w -p ./src/addons/winptyCompat\" \"tsc -w -p ./src/addons/zmodem\""
+    "watch-addons": "concurrently --kill-others-on-fail --names \"attach,fit,fullscreen,search,terminado,webLinks,winptyCompat,zmodem\" \"tsc -w -p ./src/addons/attach\" \"tsc -w -p ./src/addons/fit\" \"tsc -w -p ./src/addons/fullscreen\" \"tsc -w -p ./src/addons/search\" \"tsc -w -p ./src/addons/terminado\" \"tsc -w -p ./src/addons/webLinks\" \"tsc -w -p ./src/addons/winptyCompat\" \"tsc -w -p ./src/addons/zmodem\"",
+    "layering": "tsc -p ./src/common"
   }
 }

src/common/EventEmitter.ts

@@ -14,7 +14,7 @@ export class EventEmitter extends Disposable implements IEventEmitter, IDisposab
     super();
     // Restore the previous events if available, this will happen if the
     // constructor is called multiple times on the same object (terminal reset).
-    this._events = this._events || {};
+    this._events = (<any>this)._events || {};
   }
 
   public on(type: string, listener: XtermListener): void {

src/common/tsconfig.json

@@ -0,0 +1,18 @@
+{
+  "compilerOptions": {
+    "target": "es5",
+    "lib": [
+      "es5"
+    ],
+    "rootDir": ".",
+    "noEmit": true,
+    "strict": true,
+    "types": [
+      "../../node_modules/@types/mocha",
+      "../../"
+    ]
+  },
+  "include": [
+    "./**/*"
+  ]
+}

typings/xterm.d.ts

@@ -7,6 +7,8 @@
  * to be stable and consumed by external programs.
  */
 
+/// <reference lib="dom"/>
+
 declare module 'xterm' {
   /**
    * A string representing text font weight.