[wasm] Support asm.js modules with a single function.

test/mjsunit/wasm/asm-wasm.js

 // 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.
 
 // Flags: --validate-asm --allow-natives-syntax
 
 var stdlib = this;
 
 function assertValidAsm(func) {
   assertTrue(%IsAsmWasmCode(func));
@@ skipping 1563 matching lines @@
     } while (0xffffffff);
     if ((val>>>0) == 2147483668) {
       return 323;
     }
     return 0;
   }
   return {caller:main};
 }
 
 assertWasm(323, TestLoopsWithUnsigned);
+
+
+function TestSingleFunctionModule() {
+  "use asm";
+  function add(a, b) {
+    a = a | 0;
+    b = b | 0;
+    return (a + b) | 0;
+  }
+  return add;
+}
+
+assertEquals(7, TestSingleFunctionModule()(3, 4));