refactor(material-experimental/selection): fix strict property initialization errors

Updates the code to be compatible with strict property initialization.

Author: crisbeto

src/material-experimental/selection/selection-column.ts

@@ -86,12 +86,12 @@ export class MatSelectionColumn<T> implements OnInit, OnDestroy {
 
     this._syncColumnDefName();
   }
-  private _name: string;
+  private _name!: string;
 
-  @ViewChild(MatColumnDef, {static: true}) private readonly _columnDef: MatColumnDef;
-  @ViewChild(MatCellDef, {static: true}) private readonly _cell: MatCellDef;
+  @ViewChild(MatColumnDef, {static: true}) private readonly _columnDef!: MatColumnDef;
+  @ViewChild(MatCellDef, {static: true}) private readonly _cell!: MatCellDef;
   @ViewChild(MatHeaderCellDef, {static: true})
-  private readonly _headerCell: MatHeaderCellDef;
+  private readonly _headerCell!: MatHeaderCellDef;
 
   ngOnInit() {
     if (!this.selection && (typeof ngDevMode === 'undefined' || ngDevMode)) {

src/material-experimental/tsconfig.json

@@ -4,6 +4,7 @@
   "compilerOptions": {
     "rootDir": "..",
     "baseUrl": ".",
+    "strictPropertyInitialization": true,
     "paths": {
       "@angular/cdk/*": ["../cdk/*"],
       "@angular/material/*": ["../material/*"],