fix(compiler): throw an error for invalid provider

Closes angular#8870

modules/@angular/compiler/src/metadata_resolver.ts

@@ -15,7 +15,7 @@ import {DirectiveNormalizer} from './directive_normalizer';
 import {DirectiveResolver} from './directive_resolver';
 import {ListWrapper, StringMapWrapper} from './facade/collection';
 import {isBlank, isPresent, stringify} from './facade/lang';
-import {Identifiers, createIdentifierToken, resolveIdentifier} from './identifiers';
+import {Identifiers, resolveIdentifier} from './identifiers';
 import {CompilerInjectable} from './injectable';
 import {hasLifecycleHook} from './lifecycle_reflector';
 import {NgModuleResolver} from './ng_module_resolver';
@@ -24,7 +24,7 @@ import {ComponentStillLoadingError, LIFECYCLE_HOOKS_VALUES, ReflectorReader, ref
 import {ElementSchemaRegistry} from './schema/element_schema_registry';
 import {SummaryResolver} from './summary_resolver';
 import {getUrlScheme} from './url_resolver';
-import {MODULE_SUFFIX, SyncAsyncResult, SyntaxError, ValueTransformer, visitValue} from './util';
+import {MODULE_SUFFIX, SyntaxError, ValueTransformer, visitValue} from './util';
 
 export type ErrorCollector = (error: any, type?: any) => void;
 export const ERROR_COLLECTOR_TOKEN = new OpaqueToken('ErrorCollector');
@@ -779,6 +779,7 @@ export class CompileMetadataResolver {
         provider = resolveForwardRef(provider);
         let providerMeta: cpl.ProviderMeta;
         if (provider && typeof provider == 'object' && provider.hasOwnProperty('provide')) {
+          this._validate(provider);
           providerMeta = new cpl.ProviderMeta(provider.provide, provider);
         } else if (isValidType(provider)) {
           providerMeta = new cpl.ProviderMeta(provider, {useClass: provider});
@@ -812,6 +813,20 @@ export class CompileMetadataResolver {
     return compileProviders;
   }
 
+  private _validate(provider: any): void {
+    if (provider.hasOwnProperty('useClass') && provider.useClass == null) {
+      throw new Error(`
+          Invalid provider for ${stringifyType(provider.token)}. useClass cannot be null or undefined, got: ${provider.useClass}.
+          Usually it happens when:
+           1. Missing import statement.
+           2. There's a circular dependency (might be caused by using index.ts files).
+           3. Class was declared after it was used: 
+
+           const provider = {provide: SomeClass, useClass: SomeClass};
+           class SomeClass {}`);
+    }
+  }
+
   private _getEntryComponentsFromProvider(provider: cpl.ProviderMeta, type?: any):
       cpl.CompileIdentifierMetadata[] {
     const components: cpl.CompileIdentifierMetadata[] = [];

modules/@angular/compiler/test/metadata_resolver_spec.ts

@@ -222,6 +222,16 @@ export function main() {
                  SyntaxError, `Can't resolve all parameters for NonAnnotatedService: (?).`);
        }));
 
+    it('should throw with descriptive error message when encounter invalid provider',
+       inject([CompileMetadataResolver], (resolver: CompileMetadataResolver) => {
+         @NgModule({providers: [{provide: SimpleService, useClass: undefined}]})
+         class SomeModule {
+         }
+
+         expect(() => resolver.loadNgModuleDirectiveAndPipeMetadata(SomeModule, true))
+             .toThrowError(/Invalid provider for SimpleService. useClass cannot be null or undefined, got: undefined.`/);
+       }));
+
     it('should throw with descriptive error message when one of providers is not present',
        inject([CompileMetadataResolver], (resolver: CompileMetadataResolver) => {
          @NgModule({declarations: [MyBrokenComp3]})