Fix circular deps in isolated bundles (e.g. workers)

Closes #1669.

src/Bundler.js

@@ -607,6 +607,12 @@ class Bundler extends EventEmitter {
       }
     }
 
+    // Skip this asset if it's already in the bundle.
+    // Happens when circular dependencies are placed in an isolated bundle (e.g. a worker).
+    if (bundle.isolated && bundle.assets.has(asset)) {
+      return;
+    }
+
     let isEntryAsset =
       asset.parentBundle && asset.parentBundle.entryAsset === asset;
 

test/integration/worker-circular/index.js

@@ -0,0 +1 @@
+new Worker('worker.js');

test/integration/worker-circular/worker-dep.js

@@ -0,0 +1 @@
+require('./worker');

test/integration/worker-circular/worker.js

@@ -0,0 +1 @@
+require('./worker-dep');

test/javascript.js

@@ -250,6 +250,22 @@ describe('javascript', function() {
     });
   });
 
+  it('should support bundling workers with circular dependencies', async function() {
+    let b = await bundle(__dirname + '/integration/worker-circular/index.js', {
+      sourceMaps: false
+    });
+
+    await assertBundleTree(b, {
+      name: 'index.js',
+      assets: ['index.js'],
+      childBundles: [
+        {
+          assets: ['worker.js', 'worker-dep.js']
+        }
+      ]
+    });
+  });
+
   it('should dynamic import files which import raw files', async function() {
     let b = await bundle(
       __dirname + '/integration/dynamic-references-raw/index.js'