[Maps] Mark instance state as readonly

x-pack/legacy/plugins/maps/public/layers/fields/es_agg_field.ts

@@ -23,11 +23,11 @@ export interface IESAggField extends IField {
 }
 
 export class ESAggField implements IESAggField {
-  private _source: IESAggSource;
-  private _origin: FIELD_ORIGIN;
-  private _label?: string;
-  private _aggType: AGG_TYPE;
-  private _esDocField?: IField | undefined;
+  private readonly _source: IESAggSource;
+  private readonly _origin: FIELD_ORIGIN;
+  private readonly _label?: string;
+  private readonly _aggType: AGG_TYPE;
+  private readonly _esDocField?: IField | undefined;
 
   constructor({
     label,

x-pack/legacy/plugins/maps/public/layers/fields/top_term_percentage_field.ts

@@ -12,7 +12,7 @@ import { TOP_TERM_PERCENTAGE_SUFFIX } from '../../../common/constants';
 import { FIELD_ORIGIN } from '../../../common/constants';
 
 export class TopTermPercentageField implements IESAggField {
-  private _topTermAggField: IESAggField;
+  private readonly _topTermAggField: IESAggField;
 
   constructor(topTermAggField: IESAggField) {
     this._topTermAggField = topTermAggField;

x-pack/legacy/plugins/maps/public/layers/styles/vector/properties/style_property.ts

@@ -34,8 +34,8 @@ export interface IStyleProperty {
 }
 
 export class AbstractStyleProperty implements IStyleProperty {
-  private _options: StylePropertyOptions;
-  private _styleName: string;
+  private readonly _options: StylePropertyOptions;
+  private readonly _styleName: string;
 
   constructor(options: StylePropertyOptions, styleName: string) {
     this._options = options;

x-pack/legacy/plugins/maps/public/layers/tile_layer.test.ts

@@ -17,7 +17,7 @@ const sourceDescriptor: XYZTMSSourceDescriptor = {
 };
 
 class MockTileSource implements ITMSSource {
-  private _descriptor: XYZTMSSourceDescriptor;
+  private readonly _descriptor: XYZTMSSourceDescriptor;
   constructor(descriptor: XYZTMSSourceDescriptor) {
     this._descriptor = descriptor;
   }