flutter pub upgrade (#524)

web/github_dataviz/pubspec.lock

@@ -1,6 +1,13 @@
 # Generated by pub
 # See https://dart.dev/tools/pub/glossary#lockfile
 packages:
+  characters:
+    dependency: transitive
+    description:
+      name: characters
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.0.0"
   charcode:
     dependency: transitive
     description:
@@ -14,7 +21,7 @@ packages:
       name: collection
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.14.12"
+    version: "1.14.13"
   flutter:
     dependency: "direct main"
     description: flutter
@@ -26,7 +33,7 @@ packages:
       name: http
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.12.1"
+    version: "0.12.2"
   http_parser:
     dependency: transitive
     description:
@@ -94,7 +101,7 @@ packages:
       name: typed_data
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.6"
+    version: "1.2.0"
   vector_math:
     dependency: transitive
     description:
@@ -103,4 +110,4 @@ packages:
     source: hosted
     version: "2.0.8"
 sdks:
-  dart: ">=2.6.0 <3.0.0"
+  dart: ">=2.9.0-14.0.dev <3.0.0"