Faster uglify sourcemaps

Use raw mappings rather than generating and parsing the sourcemap from uglify.

src/transforms/uglify.js

@@ -8,12 +8,32 @@ module.exports = async function(asset) {
   let source = (await asset.generate()).js;
 
   let customConfig = await asset.getConfig(['.uglifyrc']);
+  let sourceMap = asset.options.sourceMap && new SourceMap();
   let options = {
     warnings: true,
     mangle: {
       toplevel: true
     },
-    sourceMap: asset.options.sourceMaps ? {filename: asset.relativeName} : false
+    output: sourceMap
+      ? {
+          source_map: {
+            add(source, gen_line, gen_col, orig_line, orig_col, name) {
+              sourceMap.addMapping({
+                source,
+                name,
+                original: {
+                  line: orig_line,
+                  column: orig_col
+                },
+                generated: {
+                  line: gen_line,
+                  column: gen_col
+                }
+              });
+            }
+          }
+        }
+      : {}
   };
 
   if (customConfig) {
@@ -26,15 +46,14 @@ module.exports = async function(asset) {
     throw result.error;
   }
 
-  if (result.map) {
-    result.map = await new SourceMap().addMap(JSON.parse(result.map));
+  if (sourceMap) {
     if (asset.sourceMap) {
       asset.sourceMap = await new SourceMap().extendSourceMap(
         asset.sourceMap,
-        result.map
+        sourceMap
       );
     } else {
-      asset.sourceMap = result.map;
+      asset.sourceMap = sourceMap;
     }
   }