Print declare for TSInterfaceDeclaration (#2574)

src/printer.js

@@ -856,6 +856,10 @@ function genericPrintNoParens(path, options, print, args) {
       ]);
     }
     case "TSInterfaceDeclaration":
+      if (isNodeStartingWithDeclare(n, options)) {
+        parts.push("declare ");
+      }
+
       parts.push(
         n.abstract ? "abstract " : "",
         printTypeScriptModifiers(path, options, print),

tests/typescript_declare/__snapshots__/jsfmt.spec.js.snap

@@ -18,3 +18,14 @@ declare namespace A {
 }
 
 `;
+
+exports[`declare_interface.ts 1`] = `
+declare interface Dictionary<T> {
+  [index: string]: T
+}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+declare interface Dictionary<T> {
+  [index: string]: T;
+}
+
+`;

tests/typescript_declare/declare_interface.ts

@@ -0,0 +1,3 @@
+declare interface Dictionary<T> {
+  [index: string]: T
+}