[wasm][asm.js] Forbid function declaration replacing variable declaration.

test/mjsunit/asm/asm-validation.js

 // Copyright 2016 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.
 
 // Flags: --validate-asm --allow-natives-syntax
 
 // Note that this test file contains tests that explicitly check modules are
 // valid asm.js and then break them with invalid instantiation arguments. If
 // this script is run more than once (e.g. --stress-opt) then modules remain
 // broken in the second run and assertions would fail. We prevent re-runs.
@@ skipping 345 matching lines @@
           return 43;
       }
       return 0;
     }
     return { foo: foo };
   }
   var m = Module();
   assertFalse(%IsAsmWasmCode(Module));
   assertEquals(43, m.foo(3));
 })();
+
+(function TestVarHidesExport() {
+  function Module() {
+    "use asm";
+    var foo;
+    function foo() {}
+    return foo;
+  }
+  Module();
+  assertFalse(%IsAsmWasmCode(Module));
+})();