fix(compiler): report a reasonable error with invalid metadata (#20062)

The compiler would throw an internal exception if an import using
the `ngModule` syntax and the module as not a resolvable symbol.

Fixes: #20049

packages/compiler/src/metadata_resolver.ts

@@ -664,6 +664,7 @@ export class CompileMetadataResolver {
   }
 
   private _getTypeDescriptor(type: Type): string {
+    if (isValidType(type)) {
       if (this.isDirective(type)) {
         return 'directive';
       }
@@ -675,6 +676,7 @@ export class CompileMetadataResolver {
       if (this.isNgModule(type)) {
         return 'module';
       }
+    }
 
     if ((type as any).provide) {
       return 'provider';

packages/compiler/test/metadata_resolver_spec.ts

@@ -399,6 +399,18 @@ export function main() {
 
          expect(() => resolver.getNgModuleMetadata(ModuleWithComponentInBootstrap)).not.toThrow();
        }));
+
+    // #20049
+    it('should throw a reasonable message when an invalid import is given',
+       inject([CompileMetadataResolver], (resolver: CompileMetadataResolver) => {
+         @NgModule({imports: [{ngModule: true as any}]})
+         class InvalidModule {
+         }
+
+         expect(() => { resolver.getNgModuleMetadata(InvalidModule); })
+             .toThrowError(
+                 `Unexpected value '[object Object]' imported by the module 'InvalidModule'. Please add a @NgModule annotation.`);
+       }));
   });
 
   it('should dedupe declarations in NgModule',