src/vs/workbench/contrib/debug/common/abstractDebugAdapter.ts/AbstractDebugAdapter/onMessage
class AbstractDebugAdapter:	onMessage(callback: (message: DebugProtocol.ProtocolMessage) => void): void {
		if (this.messageCallback) {
			this._onError.fire(new Error(`attempt to set more than one 'Message' callback`));
		}
		this.messageCallback = callback;
	}