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

test/mjsunit/asm/asm-validation.js

Index: test/mjsunit/asm/asm-validation.js
diff --git a/test/mjsunit/asm/asm-validation.js b/test/mjsunit/asm/asm-validation.js
index 0c9e841fe94ad4b95ac0579b482cf5ed9b41ccb5..ab675aded253d4fec0e3abed5a12f6062b6a72ba 100644
--- a/test/mjsunit/asm/asm-validation.js
+++ b/test/mjsunit/asm/asm-validation.js
@@ -363,3 +363,14 @@ function assertValidAsm(func) {
   assertFalse(%IsAsmWasmCode(Module));
   assertEquals(43, m.foo(3));
 })();
+
+(function TestVarHidesExport() {
+  function Module() {
+    "use asm";
+    var foo;
+    function foo() {}
+    return foo;
+  }
+  Module();
+  assertFalse(%IsAsmWasmCode(Module));
+})();