Fix sveltejs#3634: Add left hand side of compound assignment as depen…

…dency

Compound assignments ('x += y', 'x *= y', etc) should have the
left-hand side of the assignment as a dependency.

src/compiler/compile/Component.ts

@@ -46,6 +46,9 @@ childKeys.EachBlock = childKeys.IfBlock = ['children', 'else'];
 childKeys.Attribute = ['value'];
 childKeys.ExportNamedDeclaration = ['declaration', 'specifiers'];
 
+// There is no datatype that describes only the compound operators so we create this list here
+const compoundAssignmentOperators = ["+=" , "-=" ,"*=", "/=" ,"%=" ,"**=", "<<=" ,">>=", ">>>=", "|=" ,"^=" ,"&="];
+
 function remove_node(
 	code: MagicString,
 	start: number,
@@ -1265,10 +1268,18 @@ export default class Component {
 						}
 
 						if (node.type === 'AssignmentExpression') {
-							extract_identifiers(get_object(node.left)).forEach(node => {
+							const left = get_object(node.left);
+
+							extract_identifiers(left).forEach(node => {
 								assignee_nodes.add(node);
 								assignees.add(node.name);
 							});
+
+
+
+							if (compoundAssignmentOperators.findIndex(op => op === node.operator) !== -1) {
+								dependencies.add(left.name);
+							}
 						} else if (node.type === 'UpdateExpression') {
 							const identifier = get_object(node.argument);
 							assignees.add(identifier.name);