Add the Omit helper type.

src/lib/es5.d.ts

@@ -1443,6 +1443,11 @@ type Exclude<T, U> = T extends U ? never : T;
  */
 type Extract<T, U> = T extends U ? T : never;
 
+/**
+ * Construct a type with the properties of T except for those in type K.
+ */
+type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>;
+
 /**
  * Exclude null and undefined from T
  */