diff --git a/dart/freeform-gradient/nix/android.nix b/dart/freeform-gradient/nix/android.nix
index 4452e18..bbdb2dd 100644
--- a/dart/freeform-gradient/nix/android.nix
+++ b/dart/freeform-gradient/nix/android.nix
@@ -2,21 +2,17 @@
 
 androidenv.composeAndroidPackages {
   toolsVersion = "25.2.5";
-  platformToolsVersion = "28.0.1";
+  platformToolsVersion = "29.0.6";
   buildToolsVersions = [ "28.0.3" ];
   includeEmulator = true;
-  emulatorVersion = "28.0.14";
+  emulatorVersion = "30.0.3";
   platformVersions = [ "28" ];
   includeSources = false;
   includeDocs = false;
   includeSystemImages = true;
   systemImageTypes = [ "default" ];
   abiVersions = [ "x86_64" ];
-  # lldbVersions = [ "2.0.2558144" ];
-  # cmakeVersions = [ "3.6.4111459" ];
   includeNDK = false;
-  # ndkVersion = "16.1.4479499";
   useGoogleAPIs = false;
   useGoogleTVAddOns = false;
-  # includeExtras = [ "extras;google;gcm" ];
 }
diff --git a/dart/freeform-gradient/nix/sources.json b/dart/freeform-gradient/nix/sources.json
index 2ad18c0..f9ff1a4 100644
--- a/dart/freeform-gradient/nix/sources.json
+++ b/dart/freeform-gradient/nix/sources.json
@@ -5,10 +5,10 @@
         "homepage": "https://github.com/nmattia/niv",
         "owner": "nmattia",
         "repo": "niv",
-        "rev": "f73bf8d584148677b01859677a63191c31911eae",
-        "sha256": "0jlmrx633jvqrqlyhlzpvdrnim128gc81q5psz2lpp2af8p8q9qs",
+        "rev": "9d35b9e4837ab88517210b1701127612c260eccf",
+        "sha256": "0q50xhnm8g2yfyakrh0nly4swyygxpi0a8cb9gp65wcakcgvzvdh",
         "type": "tarball",
-        "url": "https://github.com/nmattia/niv/archive/f73bf8d584148677b01859677a63191c31911eae.tar.gz",
+        "url": "https://github.com/nmattia/niv/archive/9d35b9e4837ab88517210b1701127612c260eccf.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "nixpkgs": {
@@ -17,10 +17,10 @@
         "homepage": "https://github.com/NixOS/nixpkgs",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "696274fae7202740e9b209ea2dfb4d0ba436751f",
-        "sha256": "0ywd9nfy1sd801b2iarjijzrk520mrxp0r5mfvwqz1l0r2018qbk",
+        "rev": "7ef527cff856ea7938dba20769a6d59ebc9575e6",
+        "sha256": "0zhvly0b99846x1y3jyva79amf0kyi9c6lwg8l3ghig669kxlwa7",
         "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs/archive/696274fae7202740e9b209ea2dfb4d0ba436751f.tar.gz",
+        "url": "https://github.com/NixOS/nixpkgs/archive/7ef527cff856ea7938dba20769a6d59ebc9575e6.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     }
 }
diff --git a/dart/freeform-gradient/nix/sources.nix b/dart/freeform-gradient/nix/sources.nix
index 7bd0f3e..b796fff 100644
--- a/dart/freeform-gradient/nix/sources.nix
+++ b/dart/freeform-gradient/nix/sources.nix
@@ -6,25 +6,33 @@ let
   # The fetchers. fetch_<type> fetches specs of type <type>.
   #
 
-  fetch_file = pkgs: spec:
-    if spec.builtin or true then
-      builtins_fetchurl { inherit (spec) url sha256; }
-    else
-      pkgs.fetchurl { inherit (spec) url sha256; };
+  fetch_file = pkgs: name: spec:
+    let
+      name' = sanitizeName name + "-src";
+    in
+      if spec.builtin or true then
+        builtins_fetchurl { inherit (spec) url sha256; name = name'; }
+      else
+        pkgs.fetchurl { inherit (spec) url sha256; name = name'; };
 
   fetch_tarball = pkgs: name: spec:
     let
-      ok = str: ! builtins.isNull (builtins.match "[a-zA-Z0-9+-._?=]" str);
-      # sanitize the name, though nix will still fail if name starts with period
-      name' = stringAsChars (x: if ! ok x then "-" else x) "${name}-src";
+      name' = sanitizeName name + "-src";
     in
       if spec.builtin or true then
         builtins_fetchTarball { name = name'; inherit (spec) url sha256; }
       else
         pkgs.fetchzip { name = name'; inherit (spec) url sha256; };
 
-  fetch_git = spec:
-    builtins.fetchGit { url = spec.repo; inherit (spec) rev ref; };
+  fetch_git = name: spec:
+    let
+      ref =
+        if spec ? ref then spec.ref else
+          if spec ? branch then "refs/heads/${spec.branch}" else
+            if spec ? tag then "refs/tags/${spec.tag}" else
+              abort "In git source '${name}': Please specify `ref`, `tag` or `branch`!";
+    in
+      builtins.fetchGit { url = spec.repo; inherit (spec) rev; inherit ref; };
 
   fetch_local = spec: spec.path;
 
@@ -40,11 +48,21 @@ let
   # Various helpers
   #
 
+  # https://github.com/NixOS/nixpkgs/pull/83241/files#diff-c6f540a4f3bfa4b0e8b6bafd4cd54e8bR695
+  sanitizeName = name:
+    (
+      concatMapStrings (s: if builtins.isList s then "-" else s)
+        (
+          builtins.split "[^[:alnum:]+._?=-]+"
+            ((x: builtins.elemAt (builtins.match "\\.*(.*)" x) 0) name)
+        )
+    );
+
   # The set of packages used when specs are fetched using non-builtins.
-  mkPkgs = sources:
+  mkPkgs = sources: system:
     let
       sourcesNixpkgs =
-        import (builtins_fetchTarball { inherit (sources.nixpkgs) url sha256; }) {};
+        import (builtins_fetchTarball { inherit (sources.nixpkgs) url sha256; }) { inherit system; };
       hasNixpkgsPath = builtins.any (x: x.prefix == "nixpkgs") builtins.nixPath;
       hasThisAsNixpkgsPath = <nixpkgs> == ./.;
     in
@@ -64,15 +82,24 @@ let
 
     if ! builtins.hasAttr "type" spec then
       abort "ERROR: niv spec ${name} does not have a 'type' attribute"
-    else if spec.type == "file" then fetch_file pkgs spec
+    else if spec.type == "file" then fetch_file pkgs name spec
     else if spec.type == "tarball" then fetch_tarball pkgs name spec
-    else if spec.type == "git" then fetch_git spec
+    else if spec.type == "git" then fetch_git name spec
     else if spec.type == "local" then fetch_local spec
     else if spec.type == "builtin-tarball" then fetch_builtin-tarball name
     else if spec.type == "builtin-url" then fetch_builtin-url name
     else
       abort "ERROR: niv spec ${name} has unknown type ${builtins.toJSON spec.type}";
 
+  # If the environment variable NIV_OVERRIDE_${name} is set, then use
+  # the path directly as opposed to the fetched source.
+  replace = name: drv:
+    let
+      saneName = stringAsChars (c: if isNull (builtins.match "[a-zA-Z0-9]" c) then "_" else c) name;
+      ersatz = builtins.getEnv "NIV_OVERRIDE_${saneName}";
+    in
+      if ersatz == "" then drv else ersatz;
+
   # Ports of functions for older nix versions
 
   # a Nix version of mapAttrs if the built-in doesn't exist
@@ -89,25 +116,29 @@ let
 
   # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/strings.nix#L269
   stringAsChars = f: s: concatStrings (map f (stringToCharacters s));
+  concatMapStrings = f: list: concatStrings (map f list);
   concatStrings = builtins.concatStringsSep "";
 
+  # https://github.com/NixOS/nixpkgs/blob/8a9f58a375c401b96da862d969f66429def1d118/lib/attrsets.nix#L331
+  optionalAttrs = cond: as: if cond then as else {};
+
   # fetchTarball version that is compatible between all the versions of Nix
-  builtins_fetchTarball = { url, name, sha256 }@attrs:
+  builtins_fetchTarball = { url, name ? null, sha256 }@attrs:
     let
       inherit (builtins) lessThan nixVersion fetchTarball;
     in
       if lessThan nixVersion "1.12" then
-        fetchTarball { inherit name url; }
+        fetchTarball ({ inherit url; } // (optionalAttrs (!isNull name) { inherit name; }))
       else
         fetchTarball attrs;
 
   # fetchurl version that is compatible between all the versions of Nix
-  builtins_fetchurl = { url, sha256 }@attrs:
+  builtins_fetchurl = { url, name ? null, sha256 }@attrs:
     let
       inherit (builtins) lessThan nixVersion fetchurl;
     in
       if lessThan nixVersion "1.12" then
-        fetchurl { inherit url; }
+        fetchurl ({ inherit url; } // (optionalAttrs (!isNull name) { inherit name; }))
       else
         fetchurl attrs;
 
@@ -119,14 +150,15 @@ let
         then abort
           "The values in sources.json should not have an 'outPath' attribute"
         else
-          spec // { outPath = fetch config.pkgs name spec; }
+          spec // { outPath = replace name (fetch config.pkgs name spec); }
     ) config.sources;
 
   # The "config" used by the fetchers
   mkConfig =
-    { sourcesFile ? ./sources.json
-    , sources ? builtins.fromJSON (builtins.readFile sourcesFile)
-    , pkgs ? mkPkgs sources
+    { sourcesFile ? if builtins.pathExists ./sources.json then ./sources.json else null
+    , sources ? if isNull sourcesFile then {} else builtins.fromJSON (builtins.readFile sourcesFile)
+    , system ? builtins.currentSystem
+    , pkgs ? mkPkgs sources system
     }: rec {
       # The sources, i.e. the attribute set of spec name to spec
       inherit sources;
@@ -134,5 +166,6 @@ let
       # The "pkgs" (evaluated nixpkgs) to use for e.g. non-builtin fetchers
       inherit pkgs;
     };
+
 in
 mkSources (mkConfig {}) // { __functor = _: settings: mkSources (mkConfig settings); }
diff --git a/dart/freeform-gradient/pubspec.lock b/dart/freeform-gradient/pubspec.lock
index fcf6763..543838a 100644
--- a/dart/freeform-gradient/pubspec.lock
+++ b/dart/freeform-gradient/pubspec.lock
@@ -7,42 +7,42 @@ packages:
       name: archive
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.0.11"
+    version: "2.0.13"
   args:
     dependency: transitive
     description:
       name: args
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.5.2"
+    version: "1.6.0"
   async:
     dependency: transitive
     description:
       name: async
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.4.0"
+    version: "2.4.1"
   boolean_selector:
     dependency: transitive
     description:
       name: boolean_selector
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.0.5"
+    version: "2.0.0"
   charcode:
     dependency: transitive
     description:
       name: charcode
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.2"
+    version: "1.1.3"
   collection:
     dependency: transitive
     description:
       name: collection
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.14.11"
+    version: "1.14.12"
   convert:
     dependency: transitive
     description:
@@ -56,7 +56,7 @@ packages:
       name: crypto
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.3"
+    version: "2.1.4"
   cupertino_icons:
     dependency: "direct main"
     description:
@@ -80,7 +80,7 @@ packages:
       name: image
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.4"
+    version: "2.1.12"
   matcher:
     dependency: transitive
     description:
@@ -102,13 +102,6 @@ packages:
       url: "https://pub.dartlang.org"
     source: hosted
     version: "1.6.4"
-  pedantic:
-    dependency: transitive
-    description:
-      name: pedantic
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "1.8.0+1"
   petitparser:
     dependency: transitive
     description:
@@ -122,7 +115,7 @@ packages:
       name: quiver
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.0.5"
+    version: "2.1.3"
   sky_engine:
     dependency: transitive
     description: flutter
@@ -134,7 +127,7 @@ packages:
       name: source_span
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.5.5"
+    version: "1.7.0"
   stack_trace:
     dependency: transitive
     description:
@@ -169,7 +162,7 @@ packages:
       name: test_api
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.2.11"
+    version: "0.2.15"
   typed_data:
     dependency: transitive
     description:
@@ -190,6 +183,6 @@ packages:
       name: xml
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "3.5.0"
+    version: "3.6.1"
 sdks:
-  dart: ">=2.4.0 <3.0.0"
+  dart: ">=2.6.0 <3.0.0"