Treat function declarations in modules as lexical

Closes #714

Author: marijnh

src/statement.js

@@ -498,7 +498,7 @@ pp.parseFunction = function(node, isStatement, allowExpressionBody, isAsync) {
   if (isStatement) {
     node.id = isStatement === "nullableID" && this.type !== tt.name ? null : this.parseIdent()
     if (node.id) {
-      this.checkLVal(node.id, "var")
+      this.checkLVal(node.id, this.inModule ? "let" : "var")
     }
   }
 

test/tests-harmony.js

@@ -16237,3 +16237,5 @@ test('for ([...foo, bar].baz in qux);', {
   ],
   "sourceType": "script"
 }, {ecmaVersion: 6})
+
+testFail("var f;\nfunction f() {}", "Identifier 'f' has already been declared (2:9)", {ecmaVersion: 6, sourceType: "module"});