[wasm] Support a two-level namespace for imports.

test/mjsunit/wasm/wasm-module-builder.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.
 
 function StringRef(string) {
     this.pos = -1;
     this.string = string;
 }
 
 function DataRef(data) {
@@ skipping 60 matching lines @@
 }
 
 WasmModuleBuilder.prototype.addFunction = function(name, sig) {
     var sig_index = (typeof sig) == "number" ? sig : this.addSignature(sig);
     var func = new WasmFunctionBuilder(name, sig_index);
     func.index = this.functions.length;
     this.functions.push(func);
     return func;
 }
 
+WasmModuleBuilder.prototype.addImportWithModule = function(module, name, sig) {
+  var sig_index = (typeof sig) == "number" ? sig : this.addSignature(sig);
+  this.imports.push({module: module, name: name, sig_index: sig_index});
+  return this.imports.length - 1;
+}
+
 WasmModuleBuilder.prototype.addImport = function(name, sig) {
-    var sig_index = (typeof sig) == "number" ? sig : this.addSignature(sig);
-    this.imports.push({name: name, sig_index: sig_index});
-    return this.imports.length - 1;
+  var sig_index = (typeof sig) == "number" ? sig : this.addSignature(sig);
+  this.imports.push({module: name, name: undefined, sig_index: sig_index});
+  return this.imports.length - 1;
 }
 
 WasmModuleBuilder.prototype.addDataSegment = function(addr, data, init) {
     this.data_segments.push({addr: addr, data: data, init: init});
     return this.data_segments.length - 1;
 }
 
 WasmModuleBuilder.prototype.appendToFunctionTable = function(array) {
     this.function_table = this.function_table.concat(array);
     return this;
@@ skipping 70 matching lines @@
         }
     }
 
     // Add imports section
     if (this.imports.length > 0) {
         if (debug) print("emitting imports @ " + bytes.length);
         emit_u8(bytes, kDeclImportTable);
         emit_varint(bytes, this.imports.length);
         for (imp of this.imports) {
             emit_u16(bytes, imp.sig_index);
-            emit_string(bytes, "");
-            emit_string(bytes, imp.name);
+            emit_string(bytes, imp.module);
+            if (imp.name == undefined) {
+              emit_u32(bytes, 0);
+            } else {
+              emit_string(bytes, imp.name);
+            }
         }
     }
 
     // Add functions section
     var names = false;
     var exports = 0;
     if (this.functions.length > 0) {
         if (debug) print("emitting functions @ " + bytes.length);
         emit_u8(bytes, kDeclFunctions);
         emit_varint(bytes, this.functions.length);
@@ skipping 170 matching lines @@
 }
 
 WasmModuleBuilder.prototype.instantiate = function(ffi, memory) {
     var buffer = this.toBuffer();
     if (memory != undefined) {
       return Wasm.instantiateModule(buffer, ffi, memory);
     } else {
       return Wasm.instantiateModule(buffer, ffi);
     }
 }