Declare writeBuffer/_innerWrite access in .d.ts

src/vs/workbench/contrib/terminal/browser/xterm-private.d.ts

@@ -22,6 +22,11 @@ export interface XTermCore {
 		};
 		_onIntersectionChange: any;
 	};
+
+	// TODO: Remove below once a synchronous write API is added
+	// The below are only used in tests
+	writeBuffer: string[];
+	_innerWrite(): void;
 }
 
 export interface IEventEmitter<T> {

src/vs/workbench/contrib/terminal/test/electron-browser/terminalCommandTracker.test.ts

@@ -9,13 +9,8 @@ import { CommandTrackerAddon } from 'vs/workbench/contrib/terminal/browser/addon
 import { isWindows } from 'vs/base/common/platform';
 import { XTermCore } from 'vs/workbench/contrib/terminal/browser/xterm-private';
 
-interface TestTerminalCore extends XTermCore {
-	writeBuffer: string[];
-	_innerWrite(): void;
-}
-
 interface TestTerminal extends Terminal {
-	_core: TestTerminalCore;
+	_core: XTermCore;
 }
 
 function syncWrite(term: TestTerminal, data: string): void {