fix(forms): improve ngModel error messages (#10314)

modules/@angular/forms/src/directives/reactive_errors.ts

@@ -0,0 +1,63 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {BaseException} from '../facade/exceptions';
+import {FormErrorExamples as Examples} from './error_examples';
+
+export class ReactiveErrors {
+  static controlParentException(): void {
+    throw new BaseException(
+        `formControlName must be used with a parent formGroup directive.  You'll want to add a formGroup
+       directive and pass it an existing FormGroup instance (you can create one in your class).
+
+      Example:
+
+      ${Examples.formControlName}`);
+  }
+
+  static ngModelGroupException(): void {
+    throw new BaseException(
+        `formControlName cannot be used with an ngModelGroup parent. It is only compatible with parents
+       that also have a "form" prefix: formGroupName, formArrayName, or formGroup.
+
+       Option 1:  Update the parent to be formGroupName (reactive form strategy)
+
+        ${Examples.formGroupName}
+
+        Option 2: Use ngModel instead of formControlName (template-driven strategy)
+
+        ${Examples.ngModelGroup}`);
+  }
+  static missingFormException(): void {
+    throw new BaseException(`formGroup expects a FormGroup instance. Please pass one in.
+
+       Example:
+
+       ${Examples.formControlName}`);
+  }
+
+  static groupParentException(): void {
+    throw new BaseException(
+        `formGroupName must be used with a parent formGroup directive.  You'll want to add a formGroup
+      directive and pass it an existing FormGroup instance (you can create one in your class).
+
+      Example:
+
+      ${Examples.formGroupName}`);
+  }
+
+  static arrayParentException(): void {
+    throw new BaseException(
+        `formArrayName must be used with a parent formGroup directive.  You'll want to add a formGroup
+       directive and pass it an existing FormGroup instance (you can create one in your class).
+
+        Example:
+
+        ${Examples.formArrayName}`);
+  }
+}