[javascript mode] Handle string defaults in arrow function param lists

Closes #5934

Author: marijnh

mode/javascript/javascript.js

@@ -196,7 +196,11 @@ CodeMirror.defineMode("javascript", function(config, parserConfig) {
       } else if (wordRE.test(ch)) {
         sawSomething = true;
       } else if (/["'\/]/.test(ch)) {
-        return;
+        for (;; --pos) {
+          if (pos == 0) return
+          let next = stream.string.charAt(pos - 1)
+          if (next == ch && stream.string.charAt(pos - 2) != "\\") { pos--; break }
+        }
       } else if (sawSomething && !depth) {
         ++pos;
         break;

mode/javascript/test.js

@@ -127,6 +127,9 @@
      "[keyword let] [def f] [operator =] ([[ [def a], [def b] ]], [def c]) [operator =>] [variable-2 a] [operator +] [variable-2 c];",
      "[variable c];");
 
+  MT("fatArrow_stringDefault",
+     "([def a], [def b] [operator =] [string 'x\\'y']) [operator =>] [variable-2 a] [operator +] [variable-2 b]")
+
   MT("spread",
      "[keyword function] [def f]([def a], [meta ...][def b]) {",
      "  [variable something]([variable-2 a], [meta ...][variable-2 b]);",