fix(upgrade): throw error if trying to get injector before setting (#18209)

Previously, `undefined` would be returned.
This change makes it easier to identify incorrect uses/bugs.
(Discussed in https://github.com/angular/angular/pull/18213#issuecomment-316191308.)

packages/upgrade/src/static/angular1_providers.ts

@@ -17,6 +17,10 @@ export function setTempInjectorRef(injector: angular.IInjectorService) {
   tempInjectorRef = injector;
 }
 export function injectorFactory() {
+  if (!tempInjectorRef) {
+    throw new Error('Trying to get the AngularJS injector before it being set.');
+  }
+
   const injector: angular.IInjectorService|null = tempInjectorRef;
   tempInjectorRef = null;  // clear the value to prevent memory leaks
   return injector;

packages/upgrade/test/static/angular1_providers_spec.ts

@@ -28,12 +28,16 @@ export function main() {
         expect(injector).toBe(mockInjector);
       });
 
+      it('should throw if the injector value has not been set yet', () => {
+        const mockInjector = {get: () => {}, has: () => false};
+        expect(injectorFactory).toThrowError();
+      });
+
       it('should unset the injector after the first call (to prevent memory leaks)', () => {
         const mockInjector = {get: () => {}, has: () => false};
         setTempInjectorRef(mockInjector);
         injectorFactory();
-        const injector = injectorFactory();
-        expect(injector).toBe(null);
+        expect(injectorFactory).toThrowError();  // ...because it has been unset
       });
     });