fix(core): Provide setDevMode() to enable/disable development mode in Javascript.

modules/angular2/examples/core/ts/dev_mode/dev_mode_example.ts

@@ -0,0 +1,7 @@
+// #docregion enableDevMode
+import {bootstrap, enableDevMode} from 'angular2/core';
+import {MyComponent} from 'my_component';
+
+enableDevMode();
+bootstrap(MyComponent);
+// #enddocregion

modules/angular2/examples/core/ts/dev_mode/my_component.ts

@@ -0,0 +1,5 @@
+import {Component} from 'angular2/core';
+
+@Component({selector: 'my-component', template: '<h1>My Component</h1>'})
+export class MyComponent {
+}