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

R=rossberg@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2650053005
Cr-Commit-Position: refs/heads/master@{#42655}

test/mjsunit/wasm/js-api.js

@@ -292,19 +292,10 @@ assertErrorMessage(
 
 let customSectionModuleBinary2 = (() => {
   let builder = new WasmModuleBuilder();
-  builder.addExplicitSection([kUnknownSectionCode, 3, 1, 'x'.charCodeAt(0), 2]);
-  builder.addExplicitSection([
-    kUnknownSectionCode, 6, 3, 'f'.charCodeAt(0), 'o'.charCodeAt(0),
-    'o'.charCodeAt(0), 66, 77
-  ]);
-  builder.addExplicitSection([
-    kUnknownSectionCode, 7, 3, 'f'.charCodeAt(0), 'o'.charCodeAt(0),
-    'o'.charCodeAt(0), 91, 92, 93
-  ]);
-  builder.addExplicitSection([
-    kUnknownSectionCode, 7, 3, 'f'.charCodeAt(0), 'o'.charCodeAt(0),
-    'x'.charCodeAt(0), 99, 99, 99
-  ]);
+  builder.addCustomSection('x', [2]);
+  builder.addCustomSection('foo', [66, 77]);
+  builder.addCustomSection('foo', [91, 92, 93]);
+  builder.addCustomSection('fox', [99, 99, 99]);
   return new Int8Array(builder.toBuffer());
 })();
 var arr = moduleCustomSections(new Module(customSectionModuleBinary2), 'x');

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);