diff --git a/modules/angular2/core.ts b/modules/angular2/core.ts
index b5eb20c3f0..390ed4b5ea 100644
--- a/modules/angular2/core.ts
+++ b/modules/angular2/core.ts
@@ -20,3 +20,4 @@ export * from './src/common/forms';
 export * from './src/core/debug';
 export * from './src/core/change_detection';
 export * from './src/core/ambient';
+export * from './src/core/dev_mode';
diff --git a/modules/angular2/examples/core/ts/dev_mode/dev_mode_example.ts b/modules/angular2/examples/core/ts/dev_mode/dev_mode_example.ts
new file mode 100644
index 0000000000..d3ec8bcbdc
--- /dev/null
+++ b/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
diff --git a/modules/angular2/examples/core/ts/dev_mode/my_component.ts b/modules/angular2/examples/core/ts/dev_mode/my_component.ts
new file mode 100644
index 0000000000..e265b80648
--- /dev/null
+++ b/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 {
+}
diff --git a/modules/angular2/manual_typings/globals-es6.d.ts b/modules/angular2/manual_typings/globals-es6.d.ts
index eb8614f505..02540be14c 100644
--- a/modules/angular2/manual_typings/globals-es6.d.ts
+++ b/modules/angular2/manual_typings/globals-es6.d.ts
@@ -24,6 +24,7 @@ interface BrowserNodeGlobal {
   zone: Zone;
   getAngularTestability: Function;
   getAllAngularTestabilities: Function;
+  angularDevMode: boolean;
   setTimeout: Function;
   clearTimeout: Function;
   setInterval: Function;
diff --git a/modules/angular2/src/core/application_ref.ts b/modules/angular2/src/core/application_ref.ts
index c0ef8483c3..bde6c2d533 100644
--- a/modules/angular2/src/core/application_ref.ts
+++ b/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;
diff --git a/modules/angular2/src/core/dev_mode.dart b/modules/angular2/src/core/dev_mode.dart
new file mode 100644
index 0000000000..ad6c2091ea
--- /dev/null
+++ b/modules/angular2/src/core/dev_mode.dart
@@ -0,0 +1 @@
+// Dart development mode is determined by checked mode.
\ No newline at end of file
diff --git a/modules/angular2/src/core/dev_mode.ts b/modules/angular2/src/core/dev_mode.ts
new file mode 100644
index 0000000000..b3fa5ea65d
--- /dev/null
+++ b/modules/angular2/src/core/dev_mode.ts
@@ -0,0 +1 @@
+export {enableDevMode} from 'angular2/src/core/facade/lang';
\ No newline at end of file
diff --git a/modules/angular2/src/core/facade/lang.dart b/modules/angular2/src/core/facade/lang.dart
index 6f0887deaf..44d343d493 100644
--- a/modules/angular2/src/core/facade/lang.dart
+++ b/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));
diff --git a/modules/angular2/src/core/facade/lang.ts b/modules/angular2/src/core/facade/lang.ts
index 96c5b8393d..4a6351bf7a 100644
--- a/modules/angular2/src/core/facade/lang.ts
+++ b/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