perf: O(n²)→O(n) Map lookups for legacy `$:` reactive statements

Use Map.get/has instead of spread+find/includes when ordering and
emitting legacy reactive statements. n is the number of `$:` statements
per component.

.changeset/legacy-reactive-map-lookups.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+perf: O(n²)→O(n) Map lookups for legacy `$:` reactive statement ordering

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -1314,7 +1314,8 @@ function order_reactive_statements(unsorted_reactive_declarations) {
 	 * @returns
 	 */
 	const add_declaration = (node, declaration) => {
-		if ([...reactive_declarations.values()].includes(declaration)) return;
+		// Visited set: each ReactiveStatement is stored under exactly one LabeledStatement node
+		if (reactive_declarations.has(node)) return;
 
 		for (const binding of declaration.dependencies) {
 			if (declaration.assignments.has(binding)) continue;

packages/svelte/src/compiler/phases/3-transform/client/transform-client.js

@@ -247,11 +247,11 @@ export function client_component(analysis, options) {
 	}
 
 	for (const [node] of analysis.reactive_statements) {
-		const statement = [...state.legacy_reactive_statements].find(([n]) => n === node);
+		const statement = state.legacy_reactive_statements.get(node);
 		if (statement === undefined) {
 			throw new Error('Could not find reactive statement');
 		}
-		instance.body.push(statement[1]);
+		instance.body.push(statement);
 	}
 
 	if (analysis.reactive_statements.size > 0) {

packages/svelte/src/compiler/phases/3-transform/server/transform-server.js

@@ -148,7 +148,7 @@ export function server_component(analysis, options) {
 	const legacy_reactive_declarations = [];
 
 	for (const [node] of analysis.reactive_statements) {
-		const statement = [...state.legacy_reactive_statements].find(([n]) => n === node);
+		const statement = state.legacy_reactive_statements.get(node);
 		if (statement === undefined) {
 			throw new Error('Could not find reactive statement');
 		}
@@ -165,7 +165,7 @@ export function server_component(analysis, options) {
 			}
 		}
 
-		instance.body.push(statement[1]);
+		instance.body.push(statement);
 	}
 
 	if (legacy_reactive_declarations.length > 0) {