fix(upgrade): add try/catch when downgrading injectables

This commit improves the error thrown by the downgrade module with a more
descriptive message on why the downgrade is failing.

Closes angular#37579

packages/upgrade/src/common/src/downgrade_injectable.ts

@@ -79,8 +79,14 @@ export function downgradeInjectable(token: any, downgradedModule: string = ''):
 
     validateInjectionKey($injector, downgradedModule, injectorKey, attemptedAction);
 
-    const injector: Injector = $injector.get(injectorKey);
-    return injector.get(token);
+    try {
+      const injector: Injector = $injector.get(injectorKey);
+      return injector.get(token);
+    } catch (e) {
+      throw new Error(
+          `Trying to get the Angular injector before bootstrapping the corresponding Angular module. Injector name: ${
+              token}`);
+    }
   };
   (factory as any)['$inject'] = [$INJECTOR];
 

packages/upgrade/src/common/test/downgrade_injectable_spec.ts

@@ -48,4 +48,16 @@ describe('downgradeInjectable', () => {
     expect(factory(mockNg1Injector)).toEqual('service value');
     expect(mockNg2Injector.get).toHaveBeenCalledWith('someToken');
   });
+
+  it('should throw an error if injector is undefined', () => {
+    const factory = downgradeInjectable('someToken', 'someModule');
+    expect(factory).toEqual(jasmine.any(Function));
+    expect((factory as any).$inject).toEqual([$INJECTOR]);
+
+    const {mockNg1Injector} = setupMockInjectors('someModule');
+    const error =
+        'Trying to get the Angular injector before bootstrapping the corresponding Angular module. Injector name: someToken';
+    mockNg1Injector.get.and.throwError(error);
+    expect(() => factory(mockNg1Injector)).toThrowError(error);
+  });
 });