Merge pull request webpack#648 from loganfsmyth/devtool-map-mutation

Don't persist sourcemap changes between steps.

Author: sokra

lib/EvalSourceMapDevToolModuleTemplatePlugin.js

@@ -2,6 +2,8 @@
 	MIT License http://www.opensource.org/licenses/mit-license.php
 	Author Tobias Koppers @sokra
 */
+var clone = require("clone");
+
 var RawSource = require("webpack-core/lib/RawSource");
 var ModuleFilenameHelpers = require("./ModuleFilenameHelpers");
 
@@ -18,7 +20,9 @@ EvalSourceMapDevToolModuleTemplatePlugin.prototype.apply = function(moduleTempla
 		if(source.__EvalSourceMapDevTool_Data)
 			return source.__EvalSourceMapDevTool_Data;
 		var content = source.source();
-		var sourceMap = source.map();
+
+		// Clone the sourcemap to ensure that the mutations below do not persist.
+		var sourceMap = clone(source.map());
 		if(!sourceMap) {
 			return source;
 		}