Fix Loader type in webpack

The sourcemap parameter passed to loaders, and that loaders pass to the
callback, is a `RawSourceMap`, not a `string | Buffer`.

types/webpack/index.d.ts

@@ -9,12 +9,14 @@
 //                 Ahmed T. Ali <https://github.com/ahmed-taj>
 //                 Alan Agius <https://github.com/alan-agius4>
 //                 Spencer Elliott <https://github.com/elliottsj>
+//                 Jason Cheatham <https://github.com/jason0x43>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 /// <reference types="node" />
 
 import Tapable = require('tapable');
 import * as UglifyJS from 'uglify-js';
+import { RawSourceMap } from 'source-map';
 
 export = webpack;
 
@@ -1020,7 +1022,7 @@ declare namespace webpack {
 
     namespace loader {
         interface Loader extends Function {
-            (this: LoaderContext, source: string | Buffer, sourceMap: string | Buffer): string | Buffer | void | undefined;
+            (this: LoaderContext, source: string | Buffer, sourceMap?: RawSourceMap): string | Buffer | void | undefined;
 
             /**
              * The order of chained loaders are always called from right to left.
@@ -1040,7 +1042,7 @@ declare namespace webpack {
             raw?: boolean;
         }
 
-        type loaderCallback = (err: Error | undefined | null, content?: string | Buffer, sourceMap?: string | Buffer) => void;
+        type loaderCallback = (err: Error | undefined | null, content?: string | Buffer, sourceMap?: RawSourceMap) => void;
 
         interface LoaderContext {
             /**

types/webpack/package.json

@@ -0,0 +1,6 @@
+{
+    "private": true,
+    "dependencies": {
+        "source-map": "^0.7.0"
+    }
+}

types/webpack/webpack-tests.ts

@@ -637,7 +637,7 @@ configuration = {
     performance,
 };
 
-function loader(this: webpack.loader.LoaderContext, source: string | Buffer, sourcemap: string | Buffer): void {
+function loader(this: webpack.loader.LoaderContext, source: string | Buffer, sourcemap?: object): void {
     this.cacheable();
 
     this.async();