Use boolean and emit indeterminate only if value changed

Author: tinayuangao

src/demo-app/checkbox/checkbox-demo.html

@@ -44,4 +44,3 @@ <h1>md-checkbox: Basic Example</h1>
 </div>
 
 <h1>Nested Checklist</h1>
-<md-checkbox-demo-nested-checklist></md-checkbox-demo-nested-checklist>

src/lib/checkbox/checkbox.ts

@@ -148,7 +148,7 @@ export class MdCheckbox implements ControlValueAccessor {
   @Output() change: EventEmitter<MdCheckboxChange> = new EventEmitter<MdCheckboxChange>();
 
   /** Event emitted when the checkbox's `indeterminate` value changes. */
-  @Output() indeterminateChange: EventEmitter<Boolean> = new EventEmitter<Boolean>();
+  @Output() indeterminateChange: EventEmitter<boolean> = new EventEmitter<boolean>();
 
   /** The native `<input type="checkbox"> element */
   @ViewChild('input') _inputElement: ElementRef;
@@ -214,14 +214,17 @@ export class MdCheckbox implements ControlValueAccessor {
   }
 
   set indeterminate(indeterminate: boolean) {
+    let changed =  indeterminate != this._indeterminate;
     this._indeterminate = indeterminate;
     if (this._indeterminate) {
       this._transitionCheckState(TransitionCheckState.Indeterminate);
     } else {
       this._transitionCheckState(
           this.checked ? TransitionCheckState.Checked : TransitionCheckState.Unchecked);
     }
-    this.indeterminateChange.emit(this._indeterminate);
+    if (changed) {
+      this.indeterminateChange.emit(this._indeterminate);
+    }
   }
 
   /** The color of the button. Can be `primary`, `accent`, or `warn`. */