fix(checkbox): Handle transition when unset checkbox is interacted with

This commit fixes a bug where the initial `unchecked -> checked`
animation wasn't firing correctly when the checkbox was interacted with
from its initial state (e.g. when `[checked]` or `[(ngModel)]` isn't
bound to the component).

Fixes #183.

Closes #227

src/components/checkbox/checkbox.spec.ts

@@ -621,6 +621,29 @@ export function main() {
         let el = fixture.debugElement.query(By.css('.md-checkbox'));
         expect(el.nativeElement.className).not.toMatch(/^md\-checkbox\-anim/g);
       });
+
+      describe('when interacted with from the initial state', function() {
+        beforeEach(function(done: any) {
+          builder.createAsync(CheckboxBarebonesController).then(function(f) {
+            fixture = f;
+            controller = fixture.componentInstance;
+            fixture.detectChanges();
+          }).then(done).catch(done.fail);
+        });
+
+        it('applies a transition class when checked', function() {
+          let el = fixture.debugElement.query(By.css('.md-checkbox'));
+          click(el, fixture);
+          expect(el.nativeElement.className).toContain('md-checkbox-anim-unchecked-checked');
+        });
+
+        it('applies a transition class when made indeterminate', function() {
+          let el = fixture.debugElement.query(By.css('.md-checkbox'));
+          controller.isIndeterminate = true;
+          fixture.detectChanges();
+          expect(el.nativeElement.className).toContain('md-checkbox-anim-unchecked-indeterminate');
+        });
+      });
     });
   });
 }
@@ -783,3 +806,12 @@ class CheckboxFormcontrolController {
   directives: [MdCheckbox]
 })
 class CheckboxEndAlignedController {}
+
+@Component({
+  selector: 'checkbox-barebones-controller',
+  template: `<md-checkbox [indeterminate]="isIndeterminate"></md-checkbox>`,
+  directives: [MdCheckbox]
+})
+class CheckboxBarebonesController {
+  public isIndeterminate: boolean = false;
+}

src/components/checkbox/checkbox.ts

@@ -246,7 +246,14 @@ export class MdCheckbox implements ControlValueAccessor {
 
     switch (oldState) {
     case TransitionCheckState.Init:
-      return '';
+      // Handle edge case where user interacts with checkbox that does not have [(ngModel)] or
+      // [checked] bound to it.
+      if (newState === TransitionCheckState.Checked) {
+        animSuffix = 'unchecked-checked';
+      } else {
+        return '';
+      }
+      break;
     case TransitionCheckState.Unchecked:
       animSuffix = newState === TransitionCheckState.Checked ?
           'unchecked-checked' : 'unchecked-indeterminate';