Remove entire first line when JSON XSSI prevention seen

The source map specification says that the entire first line should be
removed when the source map string starts with the JSON XSSI preventing
string.
Fixes mozilla#278

Author: tromey

dist/source-map.debug.js

@@ -1144,6 +1144,16 @@ return /******/ (function(modules) { // webpackBootstrap
 	}
 	exports.compareByGeneratedPositionsInflated = compareByGeneratedPositionsInflated;
 
+	/**
+	 * Strip any JSON XSSI avoidance prefix from the string (as documented
+	 * in the source maps specification), and then parse the string as
+	 * JSON.
+	 */
+	function parseSourceMapInput(str) {
+	  return JSON.parse(str.replace(/^\)]}'[^\n]*\n/, ''));
+	}
+	exports.parseSourceMapInput = parseSourceMapInput;
+
 
 /***/ }),
 /* 5 */
@@ -1377,7 +1387,7 @@ return /******/ (function(modules) { // webpackBootstrap
 	function SourceMapConsumer(aSourceMap) {
 	  var sourceMap = aSourceMap;
 	  if (typeof aSourceMap === 'string') {
-	    sourceMap = JSON.parse(aSourceMap.replace(/^\)\]\}'/, ''));
+	    sourceMap = util.parseSourceMapInput(aSourceMap);
 	  }
 
 	  return sourceMap.sections != null
@@ -1652,7 +1662,7 @@ return /******/ (function(modules) { // webpackBootstrap
 	function BasicSourceMapConsumer(aSourceMap) {
 	  var sourceMap = aSourceMap;
 	  if (typeof aSourceMap === 'string') {
-	    sourceMap = JSON.parse(aSourceMap.replace(/^\)\]\}'/, ''));
+	    sourceMap = util.parseSourceMapInput(aSourceMap);
 	  }
 
 	  var version = util.getArg(sourceMap, 'version');
@@ -2205,7 +2215,7 @@ return /******/ (function(modules) { // webpackBootstrap
 	function IndexedSourceMapConsumer(aSourceMap) {
 	  var sourceMap = aSourceMap;
 	  if (typeof aSourceMap === 'string') {
-	    sourceMap = JSON.parse(aSourceMap.replace(/^\)\]\}'/, ''));
+	    sourceMap = util.parseSourceMapInput(aSourceMap);
 	  }
 
 	  var version = util.getArg(sourceMap, 'version');