perf(forms): make FormBuilder class tree-shakable (angular#41126)

This commit makes the `FormBuilder` class tree-shakable by adding the `providedIn` property to its `@Injectable`
decorator. Now if the `FormBuilder` class is not referenced in application's code, it should not be included into
its production bundle.

PR Close angular#41126

Author: AndrewKushnir

packages/forms/src/form_builder.ts

@@ -9,6 +9,7 @@
 import {Injectable} from '@angular/core';
 
 import {AsyncValidatorFn, ValidatorFn} from './directives/validators';
+import {ReactiveFormsModule} from './form_providers';
 import {AbstractControl, AbstractControlOptions, FormArray, FormControl, FormGroup, FormHooks} from './model';
 
 function isAbstractControlOptions(options: AbstractControlOptions|
@@ -30,7 +31,7 @@ function isAbstractControlOptions(options: AbstractControlOptions|
  *
  * @publicApi
  */
-@Injectable()
+@Injectable({providedIn: ReactiveFormsModule})
 export class FormBuilder {
   /**
    * @description

packages/forms/src/form_providers.ts

@@ -10,7 +10,6 @@ import {ModuleWithProviders, NgModule} from '@angular/core';
 
 import {InternalFormsSharedModule, NG_MODEL_WITH_FORM_CONTROL_WARNING, REACTIVE_DRIVEN_DIRECTIVES, TEMPLATE_DRIVEN_DIRECTIVES} from './directives';
 import {RadioControlRegistry} from './directives/radio_control_value_accessor';
-import {FormBuilder} from './form_builder';
 
 /**
  * Exports the required providers and directives for template-driven forms,
@@ -40,7 +39,9 @@ export class FormsModule {
  */
 @NgModule({
   declarations: [REACTIVE_DRIVEN_DIRECTIVES],
-  providers: [FormBuilder, RadioControlRegistry],
+  // Note: FormBuilder is also provided in this module as a tree-shakable provider,
+  // see packages/forms/src/form_builder.ts.
+  providers: [RadioControlRegistry],
   exports: [InternalFormsSharedModule, REACTIVE_DRIVEN_DIRECTIVES]
 })
 export class ReactiveFormsModule {