refactor(core): make lockRunMode a noop and deprecate it.

BREAKING CHANGE:
- `lockRunMode` is deprecated and no more needed.

Closes #9878

modules/@angular/core/src/application_ref.ts

@@ -64,30 +64,28 @@ export function enableProdMode(): void {
   _devMode = false;
 }
 
-/**
- * Returns whether Angular is in development mode.
- * This can only be read after `lockRunMode` has been called.
- *
- * By default, this is true, unless a user calls `enableProdMode`.
- *
- * @experimental APIs related to application bootstrap are currently under review.
- */
-export function isDevMode(): boolean {
-  if (!_runModeLocked) {
-    throw new BaseException(`Dev mode can't be read before bootstrap!`);
-  }
-  return _devMode;
-}
-
 /**
  * Locks the run mode of Angular. After this has been called,
  * it can't be changed any more. I.e. `isDevMode()` will always
  * return the same value.
  *
- * @experimental APIs related to application bootstrap are currently under review.
+ * @deprecated This is a noop now. {@link isDevMode} automatically locks the run mode on first call.
  */
 export function lockRunMode(): void {
+  console.warn('lockRunMode() is deprecated and not needed any more.');
+}
+
+/**
+ * Returns whether Angular is in development mode. After called once,
+ * the value is locked and won't change any more.
+ *
+ * By default, this is true, unless a user calls `enableProdMode` before calling this.
+ *
+ * @experimental APIs related to application bootstrap are currently under review.
+ */
+export function isDevMode(): boolean {
   _runModeLocked = true;
+  return _devMode;
 }
 
 /**
@@ -104,7 +102,6 @@ export function createPlatform(injector: Injector): PlatformRef {
     throw new BaseException(
         'There can be only one platform. Destroy the previous one to create a new one.');
   }
-  lockRunMode();
   _inPlatformCreate = true;
   try {
     _platform = injector.get(PlatformRef);