[javascript mode] Be more conservative about detecting fat arrow syntax

So that => in a string or comment doesn't trigger weird indentation.

Closes #2993

mode/javascript/javascript.js

@@ -205,6 +205,8 @@ CodeMirror.defineMode("javascript", function(config, parserConfig) {
         ++depth;
       } else if (wordRE.test(ch)) {
         sawSomething = true;
+      } else if (/["'\/]/.test(ch)) {
+        return;
       } else if (sawSomething && !depth) {
         ++pos;
         break;

mode/javascript/test.js

@@ -61,6 +61,12 @@
      "    [keyword yield] [variable-2 i];",
      "}");
 
+  MT("quotedStringAddition",
+     "[keyword let] [variable f] [operator =] [variable a] [operator +] [string 'fatarrow'] [operator +] [variable c];");
+
+  MT("quotedFatArrow",
+     "[keyword let] [variable f] [operator =] [variable a] [operator +] [string '=>'] [operator +] [variable c];");
+
   MT("fatArrow",
      "[variable array].[property filter]([def a] [operator =>] [variable-2 a] [operator +] [number 1]);",
      "[variable a];", // No longer in scope