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

test/mjsunit/wasm/wasm-module-builder.js

Index: test/mjsunit/wasm/wasm-module-builder.js
diff --git a/test/mjsunit/wasm/wasm-module-builder.js b/test/mjsunit/wasm/wasm-module-builder.js
index 5fade5bc8dbda288463539b565bf9342211e299a..1e05a52a6504f4d7e3ea4be315d80d951640f53c 100644
--- a/test/mjsunit/wasm/wasm-module-builder.js
+++ b/test/mjsunit/wasm/wasm-module-builder.js
@@ -160,6 +160,22 @@ class WasmModuleBuilder {
     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);