diff --git a/modules/@angular/core/src/change_detection/change_detection_util.ts b/modules/@angular/core/src/change_detection/change_detection_util.ts
index 8196c1539c730..98ba03245c066 100644
--- a/modules/@angular/core/src/change_detection/change_detection_util.ts
+++ b/modules/@angular/core/src/change_detection/change_detection_util.ts
@@ -71,15 +71,10 @@ export class ValueUnwrapper {
  * @stable
  */
 export class SimpleChange {
-  constructor(
-      public previousValue: any, public currentValue: any, _isFirstChange: boolean = false) {
-    // Store this in a non declared field
-    // to prevent a breaking change (users might have `implement`ed SimpleChange before)
-    (<any>this)._firstChange = _isFirstChange;
-  }
+  constructor(public previousValue: any, public currentValue: any, public firstChange: boolean) {}
 
   /**
    * Check whether the new value is the first value assigned.
    */
-  isFirstChange(): boolean { return (<any>this)._firstChange; }
+  isFirstChange(): boolean { return this.firstChange; }
 }
diff --git a/tools/public_api_guard/core/index.d.ts b/tools/public_api_guard/core/index.d.ts
index 8cf62cabb2313..b1d0a9da3163f 100644
--- a/tools/public_api_guard/core/index.d.ts
+++ b/tools/public_api_guard/core/index.d.ts
@@ -842,8 +842,9 @@ export declare function setTestabilityGetter(getter: GetTestability): void;
 /** @stable */
 export declare class SimpleChange {
     currentValue: any;
+    firstChange: boolean;
     previousValue: any;
-    constructor(previousValue: any, currentValue: any, _isFirstChange?: boolean);
+    constructor(previousValue: any, currentValue: any, firstChange: boolean);
     isFirstChange(): boolean;
 }