Fix sourceMappingURL for bundles with multiple entry points

packages/core/integration-tests/test/integration/sourcemap-sourcemappingurl-multiple-entrypoints/a/index.js

@@ -0,0 +1 @@
+console.log('hello1');

packages/core/integration-tests/test/integration/sourcemap-sourcemappingurl-multiple-entrypoints/b/index.js

@@ -0,0 +1 @@
+console.log('hello2');

packages/core/integration-tests/test/integration/sourcemap-sourcemappingurl/index.js

@@ -0,0 +1 @@
+console.log('hello');

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

@@ -314,4 +314,34 @@ describe('sourcemaps', function() {
     );
     mapValidator(jsOutput, map);
   });
+
+  it('should create correct sourceMappingURL', async function() {
+    const b = await bundle(
+      path.join(__dirname, '/integration/sourcemap-sourcemappingurl/index.js')
+    );
+
+    const jsOutput = await fs.readFile(b.name, 'utf8');
+    assert(jsOutput.includes('//# sourceMappingURL=/index.js.map'));
+  });
+
+  it('should create correct sourceMappingURL with multiple entrypoints', async function() {
+    const b = await bundle([
+      path.join(
+        __dirname,
+        '/integration/sourcemap-sourcemappingurl-multiple-entrypoints/a/index.js'
+      ),
+      path.join(
+        __dirname,
+        '/integration/sourcemap-sourcemappingurl-multiple-entrypoints/b/index.js'
+      )
+    ]);
+
+    const bundle1 = [...b.childBundles][0];
+    const bundle2 = [...b.childBundles][1];
+    const jsOutput1 = await fs.readFile(bundle1.name, 'utf8');
+    const jsOutput2 = await fs.readFile(bundle2.name, 'utf8');
+
+    assert(jsOutput1.includes('//# sourceMappingURL=/a/index.js.map'));
+    assert(jsOutput2.includes('//# sourceMappingURL=/b/index.js.map'));
+  });
 });

packages/core/parcel-bundler/src/packagers/JSPackager.js

@@ -239,7 +239,7 @@ class JSPackager extends Packager {
       if (mapBundle) {
         let mapUrl = urlJoin(
           this.options.publicURL,
-          path.basename(mapBundle.name)
+          path.relative(this.options.outDir, mapBundle.name)
         );
         await this.write(`\n//# sourceMappingURL=${mapUrl}`);
       }