Enable strict null checks and restrict DOM access in common

Part of #1507
Part of #1319

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": [
+    "./**/*"
+  ]
+}