[gnome-calculator] snap: Don't use core18 yet

[Jeremy Bicha <jbicha src gnome org>]

commit 074c662d9768c0e2445dd902afdfdc1e7200088b
Author: Jeremy Bicha <jbicha ubuntu com>
Date:   Wed Jan 30 19:23:06 2019 -0500

    snap: Don't use core18 yet
    
    It's not very practical yet to use core18 in our CI.
    Ubuntu isn't yet ready to switch its seeded snaps to core18.
    gnome-calculator is a seeded snap (one of the snaps
    installed by default in Ubuntu).

 snap/snapcraft.yaml | 21 +++------------------
 1 file changed, 3 insertions(+), 18 deletions(-)
---
diff --git a/snap/snapcraft.yaml b/snap/snapcraft.yaml
index 1b5a3390..a714318d 100644
--- a/snap/snapcraft.yaml
+++ b/snap/snapcraft.yaml
@@ -20,15 +20,14 @@ description: |
   decimal, and hexadecimal), boolean algebra, one’s and two’s complementation,
   character to character code conversion, and more.
 
-grade: devel # must be 'stable' to release into candidate/stable channels
+grade: stable # must be 'stable' to release into candidate/stable channels
 confinement: strict
-base: core18
 
 plugs:
-  gnome-3-28-1804:
+  gnome-3-26-1604:
     interface: content
     target: $SNAP/gnome-platform
-    default-provider: gnome-3-28-1804
+    default-provider: gnome-3-26-1804
   gtk-3-themes:
     interface: content
     target: $SNAP/data-dir/themes
@@ -64,18 +63,6 @@ apps:
       LD_LIBRARY_PATH: $LD_LIBRARY_PATH:$SNAP/lib/gnome-calculator
 
 parts:
-  desktop-gnome-platform:
-    source: https://github.com/ubuntu/snapcraft-desktop-helpers.git
-    source-subdir: gtk
-    plugin: make
-    make-parameters: ["FLAVOR=gtk3"]
-    build-packages:
-      - build-essential
-      - libgtk-3-dev
-    override-build: |
-      snapcraftctl build
-      mkdir -pv $SNAPCRAFT_PART_INSTALL/gnome-platform
-
   gtksourceview:
     after: [desktop-gnome-platform]
     source: https://gitlab.gnome.org/GNOME/gtksourceview.git
@@ -105,8 +92,6 @@ parts:
     meson-parameters: [--prefix=/snap/gnome-calculator/current/usr]
     organize:
       snap/gnome-calculator/current/usr: usr
-    build-environment:
-      - PKG_CONFIG_PATH: $SNAPCRAFT_STAGE/usr/lib/pkgconfig
 # overriding vapidir in build-environment doesn't seem to work
     override-build: |
       sed -i.bak -e "s|vapi_dir]|vapi_dir, '--vapidir=$SNAPCRAFT_STAGE/usr/share/vala/vapi']|g" meson.build