Fix analysis errors.

lib/src/color_names.dart

@@ -8,7 +8,7 @@ import 'utils.dart';
 import 'value.dart';
 
 /// A map from (lowercase) color names to their color values.
-final colorsByName = normalizedMap({
+final colorsByName = normalizedMap<SassColor>({
   'aliceblue': new SassColor.rgb(0xF0, 0xF8, 0xFF),
   'antiquewhite': new SassColor.rgb(0xFA, 0xEB, 0xD7),
   'aquamarine': new SassColor.rgb(0x7F, 0xFF, 0xD4),

lib/src/extend/functions.dart

@@ -22,7 +22,7 @@ import '../utils.dart';
 ///
 /// For example, `.foo` is a superselector of `:matches(.foo)`.
 final _subselectorPseudos =
-    new Set.from(['matches', 'any', 'nth-child', 'nth-last-child']);
+    new Set<String>.from(['matches', 'any', 'nth-child', 'nth-last-child']);
 
 /// Returns the contents of a [SelectorList] that matches only elements that are
 /// matched by both [complex1] and [complex2].

lib/src/functions.dart

@@ -21,7 +21,7 @@ import 'value.dart';
 final _microsoftFilterStart = new RegExp(r'^[a-zA-Z]+\s*=');
 
 /// Feature names supported by Dart sass.
-final _features = new Set.from([
+final _features = new Set<String>.from([
   "global-variable-shadowing",
   "extend-selector-pseudoclass",
   "units-level-3",

lib/src/parse/selector.dart

@@ -10,7 +10,7 @@ import '../utils.dart';
 import 'parser.dart';
 
 /// Pseudo-class selectors that take unadorned selectors as arguments.
-final _selectorPseudoClasses = new Set.from(
+final _selectorPseudoClasses = new Set<String>.from(
     ["not", "matches", "current", "any", "has", "host", "host-context"]);
 
 /// A parser for selectors.