Support `@abstract` JSDoc tag

TypeScript Version: nightly (2.5.0-dev.20170712)

Code

/** @abstract */
class C {}
new C();

Expected behavior:

Error.

Actual behavior:

No error.

[tschaub]

Adding a bit more to this, it would be nice if @abstract on methods were supported.

For example, with this code:

/**
 * @abstract
 */
class Integer {
  /**
   * @abstract
   * @return {number} A number.
   */
  get() {}
}
 
class FortyTwo extends Integer {
  get() {
    return 42
  }
}


/**
 * @returns {number} The return.
 */
function getFortyTwo() {
  const number = new FortyTwo();
  return number.get();
}

tsc --pretty (at 3.1.0-dev.20180901) results in this error:

main.js:8:15 - error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.

8    * @return {number} A number.
                ~~~~~~

[ShikChen]

When trying to migrate the type checker of a large JS codebase from Closure Compiler to Typescript, we found that lacking @abstract support is the major blocker. It would be great if the PR can be merged, and we are willing to test it for our codebase if needed 😃