fix(compilation): compile under typescript 2.4 (#2780)

TypeScript 2.4 is more strict about assignability with generics,
and in some cases (when calling a generic function) it will infer
that the generic parameter is <{}> rather than the <T> that rxjs
typically wants.

Fix this by explicitly passing <T> in the cases where compilation
fails.

src/observable/FromObservable.ts

@@ -91,7 +91,7 @@ export class FromObservable<T> extends Observable<T> {
         return new FromObservable<T>(ish, scheduler);
       } else if (isArray(ish)) {
         return new ArrayObservable<T>(ish, scheduler);
-      } else if (isPromise(ish)) {
+      } else if (isPromise<T>(ish)) {
         return new PromiseObservable<T>(ish, scheduler);
       } else if (typeof ish[Symbol_iterator] === 'function' || typeof ish === 'string') {
         return new IteratorObservable<T>(ish, scheduler);

src/operator/catch.ts

@@ -66,7 +66,7 @@ import { subscribeToResult } from '../util/subscribeToResult';
  */
 export function _catch<T, R>(this: Observable<T>, selector: (err: any, caught: Observable<T>) => ObservableInput<R>): Observable<T | R> {
   const operator = new CatchOperator(selector);
-  const caught = this.lift(operator);
+  const caught = this.lift<T>(operator);
   return (operator.caught = caught);
 }
 

src/operator/defaultIfEmpty.ts

@@ -38,7 +38,7 @@ export function defaultIfEmpty<T, R>(this: Observable<T>, defaultValue?: R): Obs
  * @owner Observable
  */
 export function defaultIfEmpty<T, R>(this: Observable<T>, defaultValue: R = null): Observable<T | R> {
-  return this.lift(new DefaultIfEmptyOperator(defaultValue));
+  return this.lift<T>(new DefaultIfEmptyOperator(defaultValue));
 }
 
 class DefaultIfEmptyOperator<T, R> implements Operator<T, T | R> {