build: support source map in build and dev scripts

scripts/dev.js

@@ -21,6 +21,7 @@ const { fuzzyMatchTarget } = require('./utils')
 const args = require('minimist')(process.argv.slice(2))
 const target = args._.length ? fuzzyMatchTarget(args._)[0] : 'vue'
 const formats = args.formats || args.f
+const sourceMap = args.sourcemap || args.s
 const commit = execa.sync('git', ['rev-parse', 'HEAD']).stdout.slice(0, 7)
 
 execa(
@@ -31,8 +32,11 @@ execa(
     [
       `COMMIT:${commit}`,
       `TARGET:${target}`,
-      `FORMATS:${formats || 'global'}`
-    ].join(',')
+      `FORMATS:${formats || 'global'}`,
+      sourceMap ? `SOURCE_MAP:true` : ``
+    ]
+      .filter(Boolean)
+      .join(',')
   ],
   {
     stdio: 'inherit'