Introduce extended mode.

test/mjsunit/harmony/block-let-crankshaft.js

 // Copyright 2011 the V8 project authors. All rights reserved.
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 // modification, are permitted provided that the following conditions are
 // met:
 //
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 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 // Flags: --harmony-scoping --allow-natives-syntax
 
+// TODO(ES6): properly activate extended mode
+"use strict";
+
 // Test that temporal dead zone semantics for function and block scoped
 // ket bindings are handled by the optimizing compiler.
 
 function f(x, b) {
   let y = (b ? y : x) + 42;
   return y;
 }
 
 function g(x, b) {
   {
@@ skipping 14 matching lines @@
   f(42, true);
 } catch (e) {
   assertInstanceof(e, ReferenceError);
 }
 
 try {
   g(42, true);
 } catch (e) {
   assertInstanceof(e, ReferenceError);
 }