Migrate to null safety (flutter#110)

- *Breaking* The `Font.merge` and `BoxEdge.merge` factory constructors
  are now static methods since a factory constructor can't return null,
  but that was the entire point of these constructors.
- Add nullable return types on methods that have a branch returning
  `null`.
- Add a lot of `?` on parameters that can't provably be non-null.
- Add a lot of `!`.
- Add overrides of `TreeNode.clone` to tighten the return type in a few
  places and allow some skipped explicit casts.
- Remove some dead code on conditional branches that never would have
  been followed, even before the migration, but weren't obvious
  statically.
- Add explicit casts.

CHANGELOG.md

@@ -1,11 +1,14 @@
-## 0.16.3-dev
+## 0.17.0-nullsafety-dev
+
+- `Font.merge` and `BoxEdge.merge` are now static methods instead of factory
+  constructors.
 
 ## 0.16.2
 
 - Added support for escape codes in identifiers.
 
 ## 0.16.1
-	
+
 - Fixed a crash caused by parsing certain calc() expressions and variables names that contain numbers.
 
 ## 0.16.0

example/call_parser.dart

@@ -14,7 +14,7 @@ const _default = css.PreprocessorOptions(
 /// CSS will allow any property/value pairs regardless of validity; all of our
 /// tests (by default) will ensure that the CSS is really valid.
 StyleSheet parseCss(String cssInput,
-    {List<css.Message> errors, css.PreprocessorOptions opts}) {
+    {List<css.Message>? errors, css.PreprocessorOptions? opts}) {
   return css.parse(cssInput, errors: errors, options: opts ?? _default);
 }