Fix all strong mode errors and warnings.

lib/src/polyfill.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of csslib.parser;
 
 /**
  * CSS polyfill emits CSS to be understood by older parsers that which do not
  * understand (var, calc, etc.).
  */
@@ skipping 152 matching lines @@
       _messages.warning("Variable is not defined.", node.span);
     }
 
     var oldExpressions = currentExpressions;
     currentExpressions = node.defaultValues;
     super.visitVarUsage(node);
     currentExpressions = oldExpressions;
   }
 
   List<Expression> resolveUsageTerminal(VarUsage usage) {
-    var result = [];
+    var result = <Expression>[];
 
     var varDef = _knownVarDefs[usage.name];
     var expressions;
     if (varDef == null) {
       // VarDefinition not found try the defaultValues.
       expressions = usage.defaultValues;
     } else {
       // Use the VarDefinition found.
       expressions = (varDef.expression as Expressions).expressions;
     }
@@ skipping 62 matching lines @@
       }
     } else {
       // Return real CSS property.
       return varDef;
     }
   }
 
   // Didn't point to a var definition that existed.
   return varDef;
 }