[dart mode] Support digit separators

Author: srawlins

mode/dart/dart.js

@@ -44,6 +44,8 @@
     blockKeywords: set(blockKeywords),
     builtin: set(builtins),
     atoms: set(atoms),
+    // clike numbers without the suffixes, and with '_' separators.
+    number: /^(?:0x[a-f\d_]+|(?:[\d_]+\.?[\d_]*|\.[\d_]+)(?:e[-+]?[\d_]+)?)/i,
     hooks: {
       "@": function(stream) {
         stream.eatWhile(/[\w\$_\.]/);