Make `let` usable as an identifier in ES6 sloppy mode.

test/mjsunit/harmony/block-early-errors.js

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 12 matching lines @@
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 // Flags: --harmony-scoping
 
 function CheckException(e) {
   var string = e.toString();
   assertInstanceof(e, SyntaxError);
-  assertTrue(string.indexOf("Illegal let") >= 0);
 }
 
 function Check(str) {
   try {
     eval("(function () { " + str + " })");
     assertUnreachable();
   } catch (e) {
     CheckException(e);
   }
   try {
     eval("(function () { { " + str + " } })");
     assertUnreachable();
   } catch (e) {
     CheckException(e);
   }
 }
 
 // Check for early syntax errors when using let
-// declarations outside of extended mode.
+// declarations outside of strict mode.
 Check("let x;");
 Check("let x = 1;");
 Check("let x, y;");