Prevent tokenizer from getting confused by yield outside of a generator

By using information the parser has. Not ideal, won't work for the
plain tokenizer, but so it goes.

Closes acornjs#552

src/expression.js

@@ -104,7 +104,12 @@ pp.parseExpression = function(noIn, refDestructuringErrors) {
 // operators like `+=`.
 
 pp.parseMaybeAssign = function(noIn, refDestructuringErrors, afterLeftParse) {
-  if (this.inGenerator && this.isContextual("yield")) return this.parseYield()
+  if (this.isContextual("yield")) {
+    if (this.inGenerator) return this.parseYield()
+    // The tokenizer will assume an expression is allowed after
+    // `yield`, but this isn't that kind of yield
+    else this.exprAllowed = false
+  }
 
   let ownDestructuringErrors = false, oldParenAssign = -1, oldTrailingComma = -1
   if (refDestructuringErrors) {

test/tests-harmony.js

@@ -16247,3 +16247,5 @@ test("function f() { var x; function x() {} }", {}, {ecmaVersion: 6, sourceType:
 test("a.of / 2", {}, {ecmaVersion: 6})
 
 test("let x = 1; x = 2", {}, {ecmaVersion: 6})
+
+test("function *f2() { () => yield / 1 }", {}, {ecmaVersion: 6})