[es6] Handle conflicts for sloppy let

src/preparser.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_PREPARSER_H
 #define V8_PREPARSER_H
 
 #include "src/v8.h"
 
 #include "src/bailout-reason.h"
@@ skipping 3752 matching lines @@
     // that duplicates are not allowed.  Of course, the arrow function may
     // itself be strict as well.
     const bool allow_duplicate_parameters = false;
     this->ValidateFormalParameters(&formals_classifier, language_mode(),
                                    allow_duplicate_parameters, CHECK_OK);
 
     // Validate strict mode.
     if (is_strict(language_mode())) {
       CheckStrictOctalLiteral(formal_parameters.scope->start_position(),
                               scanner()->location().end_pos, CHECK_OK);
+    }
+    if (is_strict(language_mode()) || allow_harmony_sloppy()) {
       this->CheckConflictingVarDeclarations(formal_parameters.scope, CHECK_OK);
     }
   }
 
   FunctionLiteralT function_literal = factory()->NewFunctionLiteral(
       this->EmptyIdentifierString(), ast_value_factory(),
       formal_parameters.scope, body, materialized_literal_count,
       expected_property_count, num_parameters,
       FunctionLiteral::kNoDuplicateParameters,
       FunctionLiteral::ANONYMOUS_EXPRESSION, FunctionLiteral::kIsFunction,
@@ skipping 195 matching lines @@
       *ok = false;
       return;
     }
     has_seen_constructor_ = true;
     return;
   }
 }
 } }  // v8::internal
 
 #endif  // V8_PREPARSER_H