fix(forms): improve error message when ngFormModel is missing a form

Closes #8136

Closes #8143

modules/angular2/src/common/forms/directives/ng_form_model.ts

@@ -1,5 +1,6 @@
-import {CONST_EXPR} from 'angular2/src/facade/lang';
+import {CONST_EXPR, isBlank} from 'angular2/src/facade/lang';
 import {ListWrapper, StringMapWrapper} from 'angular2/src/facade/collection';
+import {BaseException} from 'angular2/src/facade/exceptions';
 import {ObservableWrapper, EventEmitter} from 'angular2/src/facade/async';
 import {
   SimpleChange,
@@ -114,6 +115,7 @@ export class NgFormModel extends ControlContainer implements Form,
   }
 
   ngOnChanges(changes: {[key: string]: SimpleChange}): void {
+    this._checkFormPresent();
     if (StringMapWrapper.contains(changes, "form")) {
       var sync = composeValidators(this._validators);
       this.form.validator = Validators.compose([this.form.validator, sync]);
@@ -173,4 +175,11 @@ export class NgFormModel extends ControlContainer implements Form,
       dir.valueAccessor.writeValue(ctrl.value);
     });
   }
+
+  private _checkFormPresent() {
+    if (isBlank(this.form)) {
+      throw new BaseException(
+          `ngFormModel expects a form. Please pass one in. Example: <form [ngFormModel]="myCoolForm">`);
+    }
+  }
 }