feat(decorators): adds decorator versions of DI annotations.

In 'angular2/di' the symbol:
- Inject is a decorator
- InjectAnnotation is an annotation

Internally one an get a hold of annotations without *Annotations appened
(to make ts2dart work without workarounds) by importing from
'angular2/src/di/annotations_impl' instead of 'angular2/di'. This is
needed only for users that transpile through TS and through ts2dart.

modules/angular2/src/util/decorators.es6

@@ -0,0 +1,40 @@
+import {global} from 'angular2/src/facade/lang';
+
+export function makeDecorator(annotationCls) {
+  return function(...args) {
+    var Reflect = global.Reflect;
+    if (!(Reflect && Reflect.getMetadata)) {
+      throw 'reflect-metadata shim is required when using class decorators';
+    }
+    var annotationInstance = new annotationCls(...args);
+    return function(cls) {
+      var annotations = Reflect.getMetadata('annotations', cls);
+      annotations = annotations || [];
+      annotations.push(annotationInstance);
+      Reflect.defineMetadata('annotations', annotations, cls);
+      return cls;
+    }
+  }
+}
+
+export function makeParamDecorator(annotationCls) {
+  return function(...args) {
+    var Reflect = global.Reflect;
+    if (!(Reflect && Reflect.getMetadata)) {
+      throw 'reflect-metadata shim is required when using parameter decorators';
+    }
+    var annotationInstance = new annotationCls(...args);
+    return function(cls, unusedKey, index) {
+      var parameters = Reflect.getMetadata('parameters', cls);
+      parameters = parameters || [];
+      // there might be gaps if some in between parameters do not have annotations.
+      // we pad with nulls.
+      while (parameters.length <= index) {
+        parameters.push(null);
+      }
+      parameters[index] = annotationInstance;
+      Reflect.defineMetadata('parameters', parameters, cls);
+      return cls;
+    }
+  }
+}