Support let identifier in non-strict mode. (#3427)

This code adds a lot of checks and complexity to the code.

JerryScript-DCO-1.0-Signed-off-by: Zoltan Herczeg zherczeg.u-szeged@partner.samsung.com

tests/jerry/es2015/let13.js

@@ -0,0 +1,110 @@
+// Copyright JS Foundation and other contributors, http://js.foundation
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/* Declaring let */
+
+/* Note: l\u0065t is let */
+
+function check_syntax_error (code) {
+  try {
+    eval(code)
+    assert (false)
+  } catch (e) {
+    assert (e instanceof SyntaxError)
+  }
+}
+
+function check_strict_syntax_error (code) {
+  "use strict"
+
+  try {
+    eval(code)
+    assert (false)
+  } catch (e) {
+    assert (e instanceof SyntaxError)
+  }
+}
+
+check_syntax_error("let let = 5")
+check_syntax_error("const [let] = [1]")
+check_syntax_error("const l\u0065t = 6")
+check_syntax_error("let [l\u0065t] = [2]")
+check_syntax_error("l\\u0065t a")
+check_strict_syntax_error("var let = 5")
+check_strict_syntax_error("function let() {}")
+check_strict_syntax_error("for (let in []) ;")
+check_strict_syntax_error("l\\u0065t;")
+
+var let = 1
+assert(let === 1)
+
+var [let] = [2]
+assert(let === 2)
+
+var x = 0;
+let = [ () => x = 1 ]
+
+l\u0065t[0]()
+assert(x === 1)
+
+function f1()
+{
+  var a = 0
+
+  function let(l\u0065t) {
+    a = let
+  }
+
+  let(3)
+
+  assert(a === 3)
+}
+f1()
+
+function f2()
+{
+  var let = [1]
+
+  /* First: destructuring pattern definition */
+  let
+  [a] = [2]
+
+  assert(a === 2)
+
+  a = 0
+
+  /* Second: property access */
+  l\u0065t
+  [a] = [3]
+
+  assert(let[0][0] === 3)
+}
+f2()
+
+var arr = []
+
+for (let in ["x","y"])
+  arr.push(let)
+
+assert(arr[0] === "0")
+assert(arr[1] === "1")
+
+/* Let and arrow */
+
+for (let => 4; false ; ) ;
+
+let => 5
+
+/* Let label */
+let: break let