diff --git a/modules/angular2/src/facade/lang.dart b/modules/angular2/src/facade/lang.dart
index 9323bff237..96b2c01e2b 100644
--- a/modules/angular2/src/facade/lang.dart
+++ b/modules/angular2/src/facade/lang.dart
@@ -257,38 +257,34 @@ bool isJsObject(o) {
   return false;
 }
 
-bool _forceDevMode = true;
-bool _modeLocked = false;
-
-void lockMode() {
-  _modeLocked = true;
-}
-
-@deprecated
-void enableDevMode() {
-  if (_forceDevMode) {
-    return;
-  }
-  if (_modeLocked) {
-    throw new Exception("Cannot enable dev mode after platform setup.");
-  }
-  _forceDevMode = true;
-}
-
-void enableProdMode() {
-  if (_forceDevMode) {
-    return;
-  }
-  if (_modeLocked) {
-    throw new Exception("Cannot enable prod mode after platform setup.");
-  }
-  _forceDevMode = false;
-}
+// Functions below are noop in Dart. Imperatively controlling dev mode kills
+// tree shaking. We should only rely on `assertionsEnabled`.
+@Deprecated('Do not use this function. It is for JS only. There is no alternative.')
+void lockMode() {}
+@Deprecated('Do not use this function. It is for JS only. There is no alternative.')
+void enableDevMode() {}
+@Deprecated('Do not use this function. It is for JS only. There is no alternative.')
+void enableProdMode() {}
 
+/// Use this function to guard debugging code. When Dart is compiled in
+/// production mode, the code guarded using this function will be tree
+/// shaken away, reducing code size.
+///
+/// WARNING: DO NOT CHANGE THIS METHOD! This method is designed to have no
+/// more AST nodes than the maximum allowed by dart2js to inline it. In
+/// addition, the use of `assert` allows the compiler to statically compute
+/// the value returned by this function and tree shake conditions guarded by
+/// it.
+///
+/// Example:
+///
+/// if (assertionsEnabled()) {
+///   ...code here is tree shaken away in prod mode...
+/// }
 bool assertionsEnabled() {
   var k = false;
   assert((k = true));
-  return _forceDevMode || k;
+  return k;
 }
 
 // Can't be all uppercase as our transpiler would think it is a special directive...