fix(fromFetch): don't abort if fetch resolves

spec/observables/dom/fetch-spec.ts

@@ -115,14 +115,19 @@ describe('fromFetch', () => {
         expect(response).to.equal(OK_RESPONSE);
       },
       error: done,
-      complete: done,
+      complete: () => {
+        // Wait until the complete and the subsequent unsubscribe are finished
+        // before testing these expectations:
+        setTimeout(() => {
+          expect(MockAbortController.created).to.equal(1);
+          expect(mockFetch.calls.length).to.equal(1);
+          expect(mockFetch.calls[0].input).to.equal('/foo');
+          expect(mockFetch.calls[0].init.signal).not.to.be.undefined;
+          expect(mockFetch.calls[0].init.signal.aborted).to.be.false;
+          done();
+        }, 0);
+      }
     });
-
-    expect(MockAbortController.created).to.equal(1);
-    expect(mockFetch.calls.length).to.equal(1);
-    expect(mockFetch.calls[0].input).to.equal('/foo');
-    expect(mockFetch.calls[0].init.signal).not.to.be.undefined;
-    expect(mockFetch.calls[0].init.signal.aborted).to.be.false;
   });
 
   it('should handle Response that is not `ok`', done => {

src/internal/observable/dom/fetch.ts

@@ -55,6 +55,7 @@ export function fromFetch(input: string | Request, init?: RequestInit): Observab
     const controller = new AbortController();
     const signal = controller.signal;
     let outerSignalHandler: () => void;
+    let abortable = true;
     let unsubscribed = false;
 
     if (init) {
@@ -73,9 +74,11 @@ export function fromFetch(input: string | Request, init?: RequestInit): Observab
     }
 
     fetch(input, init).then(response => {
+      abortable = false;
       subscriber.next(response);
       subscriber.complete();
     }).catch(err => {
+      abortable = false;
       if (!unsubscribed) {
         // Only forward the error if it wasn't an abort.
         subscriber.error(err);
@@ -84,7 +87,9 @@ export function fromFetch(input: string | Request, init?: RequestInit): Observab
 
     return () => {
       unsubscribed = true;
-      controller.abort();
+      if (abortable) {
+        controller.abort();
+      }
     };
   });
 }