Revert "net: Forget lookups for canceled contexts"

This reverts commit 6a3d4be3.

Reason for revert: breaks various builds. See comments on CL 77670

Change-Id: Iaf3260319b560f49ace06af705a2114630f32063
Reviewed-on: https://go-review.googlesource.com/78515Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/lookup.go

@@ -200,7 +200,7 @@ func (r *Resolver) LookupIPAddr(ctx context.Context, host string) ([]IPAddr, err
 		// rather than waiting for the current lookup to
 		// complete. See issue 8602.
 		ctxErr := ctx.Err()
-		if ctxErr == context.Canceled || ctxErr == context.DeadlineExceeded {
+		if ctxErr == context.DeadlineExceeded {
 			lookupGroup.Forget(host)
 		}
 		err := mapErr(ctxErr)

src/net/lookup_test.go

@@ -739,25 +739,3 @@ func TestLookupNonLDH(t *testing.T) {
 		t.Fatalf("lookup error = %v, want %v", err, errNoSuchHost)
 	}
 }
-
-func TestLookupContextCancel(t *testing.T) {
-	if runtime.GOOS == "nacl" {
-		t.Skip("skip on NaCl")
-	}
-
-	ctx, ctxCancel := context.WithCancel(context.Background())
-	ctxCancel()
-
-	_, err := DefaultResolver.LookupIPAddr(ctx, "google.com")
-	if err != errCanceled {
-		testenv.SkipFlakyNet(t)
-		t.Fatalf("unexpected error: %q", err)
-	}
-
-	ctx = context.Background()
-
-	_, err = DefaultResolver.LookupIPAddr(ctx, "google.com")
-	if err != nil {
-		t.Fatalf("unexpected error: %q", err)
-	}
-}