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

modules/angular2/src/core/application_ref.ts

@@ -48,6 +48,7 @@ import {Compiler_} from "./linker/compiler";
 import {wtfLeave, wtfCreateScope, WtfScopeFn} from './profile/profile';
 import {ChangeDetectorRef} from 'angular2/src/core/change_detection/change_detector_ref';
 import {AMBIENT_DIRECTIVES, AMBIENT_PIPES} from "angular2/src/core/ambient";
+import {lockDevMode} from 'angular2/src/core/facade/lang';
 import {COMMON_DIRECTIVES, COMMON_PIPES} from "angular2/common";
 
 /**
@@ -126,6 +127,7 @@ var _platform: PlatformRef;
 
 export function platformCommon(providers?: Array<Type | Provider | any[]>,
                                initializer?: () => void): PlatformRef {
+  lockDevMode();
   if (isPresent(_platform)) {
     if (isBlank(providers)) {
       return _platform;

modules/angular2/src/core/dev_mode.dart

@@ -0,0 +1 @@
+// Dart development mode is determined by checked mode.

modules/angular2/src/core/dev_mode.ts

@@ -0,0 +1 @@
+export {enableDevMode} from 'angular2/src/core/facade/lang';

modules/angular2/src/core/facade/lang.dart

@@ -241,6 +241,14 @@ bool isJsObject(o) {
   return false;
 }
 
+void lockDevMode() {
+  // lockDevMode() has no effect in Dart.
+}
+
+void enableDevMode() {
+  // enableDevMode() has no effect in Dart.
+}
+
 bool assertionsEnabled() {
   var k = false;
   assert((k = true));

modules/angular2/src/core/facade/lang.ts

@@ -39,8 +39,34 @@ export function getTypeNameForDebugging(type: Type): string {
 export var Math = _global.Math;
 export var Date = _global.Date;
 
+var _devMode: boolean = !!_global.angularDevMode;
+var _devModeLocked: boolean = false;
+
+export function lockDevMode() {
+  _devModeLocked = true;
+}
+
+/**
+ * Enable Angular's development mode, which turns on assertions and other
+ * checks within the framework.
+ *
+ * One important assertion this enables verifies that a change detection pass
+ * does not result in additional changes to any bindings (also known as
+ * unidirectional data flow).
+ *
+ * {@example core/ts/dev_mode/dev_mode_example.ts region='enableDevMode'}
+ */
+export function enableDevMode() {
+  // TODO(alxhub): Refactor out of facade/lang as per issue #5157.
+  if (_devModeLocked) {
+    // Cannot use BaseException as that ends up importing from facade/lang.
+    throw 'Cannot enable dev mode after platform setup.';
+  }
+  _devMode = true;
+}
+
 export function assertionsEnabled(): boolean {
-  return false;
+  return _devMode;
 }
 
 // TODO: remove calls to assert in production environment