feat(change_detection): ensure that expression do not change after they have been checked

modules/facade/src/lang.dart

@@ -169,3 +169,12 @@ bool isJsObject(o) {
   return false;
 }
 
+
+assertionsEnabled() {
+  try {
+    assert(false);
+    return false;
+  } catch (e) {
+    return true;
+  }
+}

modules/facade/src/lang.es6

@@ -206,3 +206,12 @@ export function normalizeBlank(obj) {
 export function isJsObject(o):boolean {
   return o !== null && (typeof o === "function" || typeof o === "object");
 }
+
+export function assertionsEnabled() {
+  try {
+    var x:int = "string";
+    return false;
+  } catch (e) {
+    return true;
+  }
+}