[wasm] Implement correct 2-level namespace for imports.

test/mjsunit/wasm/gc-stress.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: --expose-wasm  --stress-gc
 
 load("test/mjsunit/wasm/wasm-constants.js");
 load("test/mjsunit/wasm/wasm-module-builder.js");
 
 function run(f) {
   var builder = new WasmModuleBuilder();
-  builder.addImport("f", kSig_i_i);
+  builder.addImport("m", "f", kSig_i_i);
   builder.addFunction("main", kSig_i_i)
     .addBody([
       kExprGetLocal, 0,
       kExprCallFunction, 0])
     .exportAs("main");
 
   print("module");
   var module = new WebAssembly.Module(builder.toBuffer());
 
   for (var i = 0; i < 10; i++) {
     print("  instance " + i);
-    var instance = new WebAssembly.Instance(module, {f: f});
+    var instance = new WebAssembly.Instance(module, {m: {f: f}});
     var g = instance.exports.main;
     for (var j = 0; j < 10; j++) {
       assertEquals(f(j), g(j));
     }
   }
 }
 
 (function test() {
   for (var i = 0; i < 100; i++) {
     run(x => (x + 19));
     run(x => (x - 18));
   }
 })();