Consider sibling in available assets to younger sibling for parallel deps

packages/bundlers/experimental/src/ExperimentalBundler.js

@@ -816,6 +816,7 @@ function createIdealGraph(
           parallelAvailability,
           assetsFromBundleRoot,
         );
+        parallelAvailability.add(child); //The next sibling should have older sibling available via parallel
       }
     }
   }

packages/core/integration-tests/test/html.js

@@ -2809,6 +2809,50 @@ describe('html', function () {
     });
   });
 
+  it('should share older JS sibling (script) assets to younger siblings', async function () {
+    // JS script tags are siblings to a common parent, and are marked as such by parallel dependency priority
+    // Becuase of load order any older sibling (and it's assets) are loaded before any subsequent sibling
+    // Which means no younger sibling should have to reference sibling bundles for assets in them
+    let b = await bundle(
+      path.join(
+        __dirname,
+        'integration/scope-hoisting/es6/sibling-dependencies/index.html',
+      ),
+    );
+    assertBundles(b, [
+      {
+        name: 'index.html',
+        assets: ['index.html'],
+      },
+      {
+        assets: ['a.js', 'esmodule-helpers.js'],
+      },
+      {
+        assets: ['b.js'],
+      },
+    ]);
+
+    let youngerSibling; // bundle containing younger sibling, b.js
+    let olderSibling; // bundle containing old sibling, a.js
+    b.traverseBundles(bundle => {
+      bundle.traverseAssets(asset => {
+        if (asset.filePath.includes('b.js')) {
+          youngerSibling = bundle;
+        } else if (asset.filePath.includes('a.js')) {
+          olderSibling = bundle;
+        }
+      });
+    });
+
+    assert(
+      b.getReferencedBundles(youngerSibling).filter(b => b == olderSibling)
+        .length == 0,
+    );
+
+    let res = await run(b, {output: null}, {require: false});
+    assert.equal(res.output, 'a');
+  });
+
   it('should escape quotes in inline style attributes and style tags', async function () {
     let b = await bundle(
       path.join(__dirname, 'integration/html-inline-escape/style.html'),