fix: ensure onMount correctly fires when new expressions are used (#14049)

* fix: ensure onMount correctly fires when used externally

* fix root context

* fix root context

* revert

* alternative fix

.changeset/early-hounds-float.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure onMount correctly fires when new expressions are used

packages/svelte/src/compiler/phases/2-analyze/visitors/NewExpression.js

@@ -11,5 +11,7 @@ export function NewExpression(node, context) {
 		w.perf_avoid_inline_class(node);
 	}
 
+	context.state.analysis.needs_context = true;
+
 	context.next();
 }

packages/svelte/tests/runtime-legacy/samples/onmount-external/_config.js

@@ -0,0 +1,7 @@
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, logs }) {
+		assert.deepEqual(logs, ['mounted']);
+	}
+});

packages/svelte/tests/runtime-legacy/samples/onmount-external/main.svelte

@@ -0,0 +1,13 @@
+<script module>
+	import { onMount } from 'svelte';
+
+	class MyClass {
+		constructor() {
+			onMount(() => console.log('mounted'));
+		}
+	}
+</script>
+
+<script>
+	new MyClass();
+</script>