fix(Dart): make some playground samples run with Dart Dev Compiler

Resolve all invalid field override errors, workaround current
reflection limitations in Dart Dev Compiler. todo, hello_world and
key_events samples now work with Dart Dev Compiler.

BREAKING CHANGE: remove TemplateRef.elementRef setter

Closes #6441

modules/angular2/src/platform/dom/dom_adapter.ts

@@ -30,7 +30,10 @@ export abstract class DomAdapter {
    * Maps attribute names to their corresponding property names for cases
    * where attribute name doesn't match property name.
    */
-  attrToPropMap: {[key: string]: string};
+  get attrToPropMap(): {[key: string]: string} { return this._attrToPropMap; };
+  set attrToPropMap(value: {[key: string]: string}) { this._attrToPropMap = value; };
+  /** @internal */
+  _attrToPropMap: {[key: string]: string};
 
   abstract parse(templateHtml: string);
   abstract query(selector: string): any;

modules/angular2/src/platform/dom/events/dom_events.ts

@@ -4,8 +4,6 @@ import {EventManagerPlugin, EventManager} from './event_manager';
 
 @Injectable()
 export class DomEventsPlugin extends EventManagerPlugin {
-  manager: EventManager;
-
   // This plugin should come last in the list of plugins, because it accepts all
   // events.
   supports(eventName: string): boolean { return true; }