[ ] Regression (behavior that used to work and stopped working in a new release)
[ ] Bug report 
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question

Angular version: 4.2.2

import { Component, Input, OnChanges, SimpleChanges, SimpleChange } from '@angular/core';

@Component({
  // ...
})
export class ExampleComponent implements OnChanges {
  @Input() foo: string;
  @Input() bar: number;

  ngOnChanges(changes: SimpleChanges): void {
    // BAD: We get no autocomplete for the properties on changes
    // BAD: We are not able to access changes.foo.currentValue here with dot syntax, we have to use changes["foo"].currentValue
    // BAD: Editor is not aware of type of changes["foo"].currentValue, so we get no further type checking / autocompletion
  }
}

import { Component, Input, OnChanges, SimpleChanges, SimpleChange } from '@angular/core';

interface IExampleComponentChanges extends SimpleChanges {
  foo: SimpleChange;
  bar: SimpleChange;
}

@Component({
  // ...
})
export class ExampleComponent implements OnChanges {
  @Input() foo: string;
  @Input() bar: number;

  ngOnChanges(changes: IExampleComponentChanges): void {
    // GOOD: Editor is aware of which properties are available in changes
    // GOOD: We are able to access changes.foo.currentValue here with dot syntax
    // BAD: Editor is not aware of type of changes.foo.currentValue, so we get no further type checking / autocompletion
  }
}

import { Component, Input, OnChanges, SimpleChanges, SimpleChange } from '@angular/core';

interface IExampleComponentChanges extends SimpleChanges {
  foo: SimpleChange<string>;
  bar: SimpleChange<number>;
}

@Component({
  // ...
})
export class ExampleComponent implements OnChanges {
  @Input() foo: string;
  @Input() bar: number;

  ngOnChanges(changes: IExampleComponentChanges): void {
    // GOOD: Editor is aware of which properties are available in changes
    // GOOD: We are able to access changes.foo.currentValue here with dot syntax
    // GOOD: Editor is aware of the type of changes.foo.currentValue so we get full type safety and autocompletion all the way down
  }
}

declare class SimpleChangeGeneric<T = any> extends SimpleChange {
  previousValue: T;
  currentValue: T;
  firstChange: boolean;
  constructor(previousValue: T, currentValue: T, firstChange: boolean);
  /**
   * Check whether the new value is the first value assigned.
   */
  isFirstChange(): boolean;
}

// ...
export interface IExampleComponentChanges extends SimpleChanges {
  foo: SimpleChangeGeneric<string>;
}
// ...