feat(pipes): add support for pure pipes

By default, pipes are pure. This means that an instance of a pipe will be reused and the pipe will be called only when its arguments change.

BREAKING CHANGE

Before:

@Pipe({name: 'date'}) class DatePipe {} defines an impure pipe.

After:

@Pipe({name: 'date'}) class DatePipe {} defines a pure pipe.
@Pipe({name: 'date', pure: false}) class DatePipe {} defines an impure pipe.

Closes #3966

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

@@ -59,7 +59,7 @@ var _observableStrategy = new ObservableStrategy();
  *
  * ```
  */
-@Pipe({name: 'async'})
+@Pipe({name: 'async', pure: false})
 @Injectable()
 export class AsyncPipe implements PipeTransform, PipeOnDestroy {
   _latestValue: Object = null;

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

@@ -4,14 +4,15 @@ import {Key, ResolvedBinding, Binding} from 'angular2/src/core/di';
 import {PipeMetadata} from '../metadata/directives';
 
 export class PipeBinding extends ResolvedBinding {
-  constructor(public name: string, key: Key, resolvedFactories: ResolvedFactory[],
-              multiBinding: boolean) {
+  constructor(public name: string, public pure: boolean, key: Key,
+              resolvedFactories: ResolvedFactory[], multiBinding: boolean) {
     super(key, resolvedFactories, multiBinding);
   }
 
   static createFromType(type: Type, metadata: PipeMetadata): PipeBinding {
     var binding = new Binding(type, {toClass: type});
     var rb = resolveBinding(binding);
-    return new PipeBinding(metadata.name, rb.key, rb.resolvedFactories, rb.multiBinding);
+    return new PipeBinding(metadata.name, metadata.pure, rb.key, rb.resolvedFactories,
+                           rb.multiBinding);
   }
 }

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

@@ -1,4 +1,5 @@
 import {isBlank, isPresent, BaseException, CONST, Type} from 'angular2/src/core/facade/lang';
+import {StringMapWrapper} from 'angular2/src/core/facade/collection';
 import {
   Injectable,
   OptionalMetadata,
@@ -25,11 +26,25 @@ export class ProtoPipes {
   }
 }
 
+
+
 export class Pipes implements cd.Pipes {
+  _config: StringMap<string, cd.SelectedPipe> = {};
+
   constructor(public proto: ProtoPipes, public injector: Injector) {}
 
-  get(name: string): any {
-    var b = this.proto.get(name);
-    return this.injector.instantiateResolved(b);
+  get(name: string): cd.SelectedPipe {
+    var cached = StringMapWrapper.get(this._config, name);
+    if (isPresent(cached)) return cached;
+
+    var p = this.proto.get(name);
+    var transform = this.injector.instantiateResolved(p);
+    var res = new cd.SelectedPipe(transform, p.pure);
+
+    if (p.pure) {
+      StringMapWrapper.set(this._config, name, res);
+    }
+
+    return res;
   }
 }