Add test to close #14635 (#15254)

tests/format/misc/errors/typescript/mapped-type/__snapshots__/jsfmt.spec.js.snap

@@ -0,0 +1,21 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`mapped-type.ts [babel-ts] format 1`] = `
+"Unexpected token, expected "}" (3:3)
+  1 | type Mapped = {
+  2 |   [key in keyof O]: number;
+> 3 |   extra_member: should_not_allowed
+    |   ^
+  4 | };
+  5 |"
+`;
+
+exports[`mapped-type.ts [typescript] format 1`] = `
+"A mapped type may not declare properties or methods. (3:3)
+  1 | type Mapped = {
+  2 |   [key in keyof O]: number;
+> 3 |   extra_member: should_not_allowed
+    |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  4 | };
+  5 |"
+`;

tests/format/misc/errors/typescript/mapped-type/jsfmt.spec.js

@@ -0,0 +1 @@
+run_spec(import.meta, ["babel-ts", "typescript"]);

tests/format/misc/errors/typescript/mapped-type/mapped-type.ts

@@ -0,0 +1,4 @@
+type Mapped = {
+  [key in keyof O]: number;
+  extra_member: should_not_allowed
+};