Add parser for Expression and VariableDeclaration

lib/src/ast/sass/expression.dart

@@ -2,6 +2,8 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import '../../logger.dart';
+import '../../parse/scss.dart';
 import '../../visitor/interface/expression.dart';
 import 'node.dart';
 
@@ -9,4 +11,12 @@ import 'node.dart';
 abstract class Expression implements SassNode {
   /// Calls the appropriate visit method on [visitor].
   T accept<T>(ExpressionVisitor<T> visitor);
+
+  /// Parses an expression from [contents].
+  ///
+  /// If passed, [url] is the name of the file from which [contents] comes.
+  ///
+  /// Throws a [SassFormatException] if parsing fails.
+  factory Expression.parse(String contents, {url, Logger logger}) =>
+      new ScssParser("$contents", url: url, logger: logger).parseExpression();
 }

lib/src/ast/sass/statement/variable_declaration.dart

@@ -4,6 +4,8 @@
 
 import 'package:source_span/source_span.dart';
 
+import '../../../logger.dart';
+import '../../../parse/scss.dart';
 import '../../../visitor/interface/statement.dart';
 import '../expression.dart';
 import '../statement.dart';
@@ -35,6 +37,15 @@ class VariableDeclaration implements Statement {
       : isGuarded = guarded,
         isGlobal = global;
 
+  /// Parses a variable declaration from [contents].
+  ///
+  /// If passed, [url] is the name of the file from which [contents] comes.
+  ///
+  /// Throws a [SassFormatException] if parsing fails.
+  factory VariableDeclaration.parse(String contents, {url, Logger logger}) =>
+      new ScssParser(contents, url: url, logger: logger)
+          .parseVariableDeclaration();
+
   T accept<T>(StatementVisitor<T> visitor) =>
       visitor.visitVariableDeclaration(this);
 

lib/src/parse/stylesheet.dart

@@ -80,6 +80,22 @@ abstract class StylesheetParser extends Parser {
     });
   }
 
+  Expression parseExpression() {
+    return wrapSpanFormatException(() {
+      var expression = _expression();
+      scanner.expectDone();
+      return expression;
+    });
+  }
+
+  VariableDeclaration parseVariableDeclaration() {
+    return wrapSpanFormatException(() {
+      var declaration = variableDeclaration();
+      scanner.expectDone();
+      return declaration;
+    });
+  }
+
   /// Parses a function signature of the format allowed by Node Sass's functions
   /// option and returns its name and declaration.
   ///