Adds several flutter snippets for dart-mode

snippets/dart-mode/didchangedependencies

@@ -0,0 +1,10 @@
+# -*- mode: snippet -*-
+# name: didChangeDependencies
+# key: dcd
+# group: flutter
+# --
+@override
+void didChangeDependencies() {
+  super.didChangeDependencies();
+  $0
+}

snippets/dart-mode/dispose

@@ -0,0 +1,10 @@
+# -*- mode: snippet -*-
+# name: dispose
+# key: is
+# group: flutter
+# --
+@override
+void dispose() {
+  super.dispose();
+  $0
+}

snippets/dart-mode/initstate

@@ -0,0 +1,10 @@
+# -*- mode: snippet -*-
+# name: initState
+# key: is
+# group: flutter
+# --
+@override
+void initState() {
+  super.initState();
+  $0
+}

snippets/dart-mode/statefulwidget

@@ -0,0 +1,16 @@
+# -*- mode: snippet -*-
+# name: StatefulWidget
+# key: sfw
+# group: flutter
+# --
+class ${1:Name} extends StatefulWidget {
+  @override
+  $1State createState() => $1State();
+}
+
+class $1State extends State<$1> {
+  @override
+  Widget build(BuildContext context) {
+    return Container($0);
+  }
+}

snippets/dart-mode/statelesswidget

@@ -0,0 +1,11 @@
+# -*- mode: snippet -*-
+# name: StatelessWidget
+# key: slw
+# group: flutter
+# --
+class ${1:Name} extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Container($0);
+  }
+}