refactor(Pipes): integrate review feedback

Closes #4947

modules/angular2/src/core/metadata/directives.ts

@@ -948,7 +948,7 @@ export class ComponentMetadata extends DirectiveMetadata {
 /**
  * Declare reusable pipe function.
  *
- * A "pure" pipe is only re-evaluated when any of its input or arguments changes.
+ * A "pure" pipe is only re-evaluated when either the input or any of the arguments change.
  *
  * When not specified, pipes default to being pure.
  *

modules/angular2/src/core/pipes/date_pipe.ts

@@ -21,12 +21,17 @@ import {InvalidPipeArgumentException} from './invalid_pipe_argument_exception';
 var defaultLocale: string = 'en-US';
 
 /**
- * WARNING: this pipe uses the Internationalization API.
- * Therefore it is only reliable in Chrome and Opera browsers.
- *
  * Formats a date value to a string based on the requested format.
  *
- *##Usage
+ * WARNINGS:
+ * - this pipe is marked as pure hence it will not be re-evaluated when the input is mutated.
+ *   Instead users should treat the date as an immutable object and change the reference when the
+ *   pipe needs to re-run (this is to avoid reformatting the date on every change detection run
+ *   which would be an expensive operation).
+ * - this pipe uses the Internationalization API. Therefore it is only reliable in Chrome and Opera
+ *   browsers.
+ *
+ * ## Usage
  *
  *     expression | date[:format]
  *
@@ -78,7 +83,7 @@ var defaultLocale: string = 'en-US';
  *     {{ dateObj | date:'mmss' }}        // output is '43:11'
  */
 @CONST()
-@Pipe({name: 'date', pure: false})
+@Pipe({name: 'date', pure: true})
 @Injectable()
 export class DatePipe implements PipeTransform {
   /** @internal */

modules/angular2/src/core/pipes/json_pipe.ts

@@ -5,7 +5,6 @@ import {Pipe} from 'angular2/src/core/metadata';
 
 /**
  * Implements json transforms to any object.
- * The json pipe runs all the time checking for changes on the transformed object.
  *
  * ### Example
  *

modules/angular2/test/core/pipes/date_pipe_spec.ts

@@ -24,8 +24,8 @@ export function main() {
       pipe = new DatePipe();
     });
 
-    it('should be marked as non-pure',
+    it('should be marked as pure',
-       () => { expect(new PipeResolver().resolve(DatePipe).pure).toEqual(false); });
+       () => { expect(new PipeResolver().resolve(DatePipe).pure).toEqual(true); });
 
     describe("supports", () => {
       it("should support date", () => { expect(pipe.supports(date)).toBe(true); });