[Interpreter] Use FastNewSloppyArguments when possible.

test/cctest/interpreter/test-interpreter.cc

 // Copyright 2015 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.
 
 #include "src/v8.h"
 
 #include "src/execution.h"
 #include "src/handles.h"
 #include "src/interpreter/bytecode-array-builder.h"
 #include "src/interpreter/bytecode-array-iterator.h"
@@ skipping 2241 matching lines @@
       std::make_pair("function f(a, b, c, d) {"
                      "'use strict'; return arguments[2]; }",
                      2),
       // Check arguments are mapped in sloppy mode and unmapped in strict.
       std::make_pair("function f(a, b, c, d) {"
                      "  c = b; return arguments[2]; }",
                      1),
       std::make_pair("function f(a, b, c, d) {"
                      "  'use strict'; c = b; return arguments[2]; }",
                      2),
+      // Check arguments for duplicate parameters in sloppy mode.
+      std::make_pair("function f(a, a, b) { return arguments[1]; }", 1),
       // check rest parameters
       std::make_pair("function f(...restArray) { return restArray[0]; }", 0),
       std::make_pair("function f(a, ...restArray) { return restArray[0]; }", 1),
       std::make_pair("function f(a, ...restArray) { return arguments[0]; }", 0),
       std::make_pair("function f(a, ...restArray) { return arguments[1]; }", 1),
       std::make_pair("function f(a, ...restArray) { return restArray[1]; }", 2),
       std::make_pair("function f(a, ...arguments) { return arguments[0]; }", 1),
       std::make_pair("function f(a, b, ...restArray) { return restArray[0]; }",
                      2),
   };
@@ skipping 1894 matching lines @@
     CHECK(return_value->SameValue(*tests[i].second));
   }
 
   FLAG_ignition_generators = old_flag;
 }
 
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8