Fix sources: ["0"], output the correct source

Case 1, sourcemap missing
In [terser-js](https://github.com/terser-js/terser#source-map-options),
sources are always 0 if old sourcemaps are not provided.

The value passed for sourceMap.url is only used to set
//# sourceMappingURL=out.js.map in result.code.

The value of filename is only used to set file attribute
in source map file.

In broccoli-uglify-sourcemap we know in this case we are generating
sourcemap for the file we are processing, changing 0 to the actual
file gives us the correct source.

Case2, multiple sourceMap comments
source-map-url only matches the very first magic comment so we
mistakenly thinks the js file doesn't have a valid sourcemap.

lib/get-sourcemap-content.js

@@ -0,0 +1,26 @@
+'use strict';
+
+const fs = require('fs');
+const path = require('path');
+const srcURL = require('source-map-url');
+const srcRegExpg = new RegExp(srcURL.regex, 'g');
+
+module.exports = function getSourceMapContent(src, inFile, relativePath, silent) {
+  let urls = [];
+  let match;
+  // eslint-disable-next-line no-cond-assign
+  while (match = srcRegExpg.exec(src)) {
+    urls.push(match[1] || match[2] || '');
+  }
+  if (urls.length) {
+    for (let i = urls.length - 1; i >= 0; --i) {
+      let sourceMapPath = path.join(path.dirname(inFile), urls[i]);
+      if (fs.existsSync(sourceMapPath)) {
+        return JSON.parse(fs.readFileSync(sourceMapPath));
+      }
+    }
+    if (!silent) {
+      console.warn(`[WARN] (broccoli-uglify-sourcemap) ${urls.map(u => `"${u}"`).join(', ')} referenced in "${relativePath}" could not be found`);
+    }
+  }
+};

lib/process-file.js

@@ -5,7 +5,7 @@ const defaults = require('lodash.defaultsdeep');
 const fs = require('fs');
 const mkdirp = require('mkdirp');
 const path = require('path');
-const srcURL = require('source-map-url');
+const getSourceMapContent = require('./get-sourcemap-content');
 
 const terser = require('terser');
 
@@ -38,14 +38,9 @@ module.exports = function processFile(inFile, outFile, relativePath, outDir, sil
 
     let sourceMap = { filename, url };
 
-    if (srcURL.existsIn(src)) {
-      let url = srcURL.getFrom(src);
-      let sourceMapPath = path.join(path.dirname(inFile), url);
-      if (fs.existsSync(sourceMapPath)) {
-        sourceMap.content = JSON.parse(fs.readFileSync(sourceMapPath));
-      } else if (!silent) {
-        console.warn(`[WARN] (broccoli-uglify-sourcemap) "${url}" referenced in "${relativePath}" could not be found`);
-      }
+    let content = getSourceMapContent(src, inFile, relativePath, silent);
+    if (content) {
+      sourceMap.content = content;
     }
 
     options = defaults(options, { sourceMap });
@@ -69,13 +64,25 @@ module.exports = function processFile(inFile, outFile, relativePath, outDir, sil
 
   if (options.sourceMap) {
     let newSourceMap = JSON.parse(result.map);
+    debug('[newSourceMap] %O', newSourceMap);
 
     newSourceMap.sources = newSourceMap.sources.map(function(path) {
-      // If out output file has the same name as one of our original
-      // sources, they will shadow eachother in Dev Tools. So instead we
-      // alter the reference to the upstream file.
       if (path === relativePath) {
-        path = path.replace(/\.js$/, '-orig.js');
+        // If out output file has the same name as one of our original
+        // sources, they will shadow eachother in Dev Tools. So instead we
+        // alter the reference to the upstream file.
+        return path.replace(/\.js$/, '-orig.js');
+      } else if (path === '0') {
+        // In [terser-js](https://github.com/terser-js/terser#source-map-options),
+        // sources are always 0 if old sourcemaps are not provided.
+        // The value passed for `sourceMap.url` is only used to set
+        // `//# sourceMappingURL=out.js.map` in `result.code`.
+        // The value of `filename` is only used to set `file` attribute
+        // in source map file.
+        // In broccoli-uglify-sourcemap we know in this case we are generating
+        // sourcemap for the file we are processing, changing 0 to the actual
+        // file gives us the correct source.
+        return relativePath;
       }
       return path;
     });