fix: disallow line break between async and property

packages/babel-generator/src/generators/methods.js

@@ -37,6 +37,8 @@ export function _methodHead(node: Object) {
   }
 
   if (node.async) {
+    // ensure `async` is in the same line with property name
+    this._catchUp("start", key.loc);
     this.word("async");
     this.space();
   }

packages/babel-generator/test/fixtures/regression/11870/input.js

@@ -0,0 +1,4 @@
+class Test {
+  @TestDecorator
+  async decorateMe() {}
+}

packages/babel-generator/test/fixtures/regression/11870/options.json

@@ -0,0 +1,4 @@
+{
+  "retainLines": true,
+  "plugins": [["decorators", { "decoratorsBeforeExport": true }]]
+}

packages/babel-generator/test/fixtures/regression/11870/output.js

@@ -0,0 +1,3 @@
+class Test {
+  @TestDecorator
+  async decorateMe() {}}