fix(datepicker): make datepicker view child static (#5374)

Fix a bug related to an undefined reference error when writing a
value to the datepicker before it's ngAfterContentInit hook.

Closes #5373

src/datepicker/datepicker.component.spec.ts

@@ -0,0 +1,43 @@
+import { async, ComponentFixture, TestBed } from '@angular/core/testing';
+import { BrowserAnimationsModule } from '@angular/platform-browser/animations';
+import { Component, ViewChild, OnInit } from '@angular/core';
+
+import { DatepickerModule, DatePickerComponent } from '.';
+
+@Component({
+  selector: 'test-cmp',
+  template: `<datepicker></datepicker>`
+})
+class TestComponent implements OnInit {
+  @ViewChild(DatePickerComponent, { static: true }) datepicker: DatePickerComponent;
+
+  ngOnInit(): void {
+    this.datepicker.writeValue(new Date());
+  }
+}
+
+type TestFixture = ComponentFixture<TestComponent>;
+
+describe('datepicker:', () => {
+  let fixture: TestFixture;
+
+  beforeEach(
+    async(() => TestBed.configureTestingModule({
+      declarations: [TestComponent],
+      imports: [
+        DatepickerModule.forRoot(),
+        BrowserAnimationsModule
+      ]
+    }).compileComponents())
+  );
+
+  it('should not throw undefined reference error when initializing value before content init hook',
+    () => {
+      // tslint:disable-next-line: no-floating-promises
+      expect(() => {
+        fixture = TestBed.createComponent(TestComponent);
+        fixture.detectChanges();
+      }).not.toThrowError(/^.*undefined.*$/gm);
+    }
+  );
+});

src/datepicker/datepicker.component.ts

@@ -122,7 +122,7 @@ export class DatePickerComponent implements ControlValueAccessor {
     undefined
   );
 
-  @ViewChild(DatePickerInnerComponent, { static: false })
+  @ViewChild(DatePickerInnerComponent, { static: true })
   _datePicker: DatePickerInnerComponent;
 
   /* tslint:disable-next-line: no-any*/