[wasm-test] WasmModuleBuilder.addCustomSection in the JS builder API.

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.
 
 // Used for encoding f32 and double constants to bits.
 let __buffer = new ArrayBuffer(8);
 let byte_view = new Int8Array(__buffer);
 let f32_view = new Float32Array(__buffer);
 let f64_view = new Float64Array(__buffer);
 
@@ skipping 142 matching lines @@
   addMemory(min, max, exp) {
     this.memory = {min: min, max: max, exp: exp};
     return this;
   }
 
   addExplicitSection(bytes) {
     this.explicit.push(bytes);
     return this;
   }
 
+  stringToBytes(name) {
+    var result = new Binary();
+    result.emit_u32v(name.length);
+    for (var i = 0; i < name.length; i++) {
+      result.emit_u8(name.charCodeAt(i));
+    }
+    return result;
+  }
+
+  addCustomSection(name, bytes) {
+    name = this.stringToBytes(name);
+    var length = new Binary();
+    length.emit_u32v(name.length + bytes.length);
+    this.explicit.push([0, ...length, ...name, ...bytes]);
+  }
+
   addType(type) {
     // TODO: canonicalize types?
     this.types.push(type);
     return this.types.length - 1;
   }
 
   addGlobal(local_type, mutable) {
     let glob = new WasmGlobalBuilder(this, local_type, mutable);
     glob.index = this.globals.length + this.num_imported_globals;
     this.globals.push(glob);
@@ skipping 375 matching lines @@
     }
     return buffer;
   }
 
   instantiate(ffi) {
     let module = new WebAssembly.Module(this.toBuffer());
     let instance = new WebAssembly.Instance(module, ffi);
     return instance;
   }
 }