Support url() imports.

lib/src/ast/css/import.dart

@@ -6,11 +6,14 @@ import 'package:source_span/source_span.dart';
 
 import '../../visitor/interface/css.dart';
 import 'node.dart';
+import 'value.dart';
 
 /// A plain CSS `@import`.
 class CssImport extends CssNode {
   /// The URL being imported.
-  final Uri url;
+  ///
+  /// This includes quotes.
+  final CssValue<String> url;
 
   final FileSpan span;
 

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

@@ -5,11 +5,11 @@
 import 'package:source_span/source_span.dart';
 
 import '../../../visitor/interface/statement.dart';
-import '../expression/string.dart';
+import '../interpolation.dart';
 import '../statement.dart';
 
 class PlainImportRule implements Statement {
-  final Uri url;
+  final Interpolation url;
 
   final FileSpan span;
 
@@ -18,5 +18,5 @@ class PlainImportRule implements Statement {
   /*=T*/ accept/*<T>*/(StatementVisitor/*<T>*/ visitor) =>
       visitor.visitPlainImportRule(this);
 
-  String toString() => "@import ${StringExpression.quoteText(url.toString())};";
+  String toString() => "@import $url;";
 }

lib/src/parse/stylesheet.dart

@@ -523,20 +523,27 @@ abstract class StylesheetParser extends Parser {
     }
 
     // TODO: parse supports clauses, url(), and query lists
-    var urlString = string();
-
-    Uri url;
-    try {
-      url = Uri.parse(urlString);
-    } on FormatException catch (error) {
-      throw new SassFormatException(
-          "Invalid URL: ${error.message}", scanner.spanFrom(start));
+    var urlStart = scanner.state;
+    var next = scanner.peekChar();
+    if (next == $u || next == $U) {
+      return new PlainImportRule(
+          new Interpolation([_dynamicUrl()], scanner.spanFrom(urlStart)),
+          scanner.spanFrom(start));
     }
 
-    if (_isPlainImportUrl(urlString)) {
-      return new PlainImportRule(url, scanner.spanFrom(start));
+    var url = string();
+    if (_isPlainImportUrl(url)) {
+      return new PlainImportRule(
+          new Interpolation([scanner.substring(urlStart.position)],
+              scanner.spanFrom(urlStart)),
+          scanner.spanFrom(start));
     } else {
-      return new ImportRule(url, scanner.spanFrom(start));
+      try {
+        return new ImportRule(Uri.parse(url), scanner.spanFrom(start));
+      } on FormatException catch (error) {
+        throw new SassFormatException(
+            "Invalid URL: ${error.message}", scanner.spanFrom(start));
+      }
     }
   }
 
@@ -1499,6 +1506,17 @@ abstract class StylesheetParser extends Parser {
     return null;
   }
 
+  Expression _dynamicUrl() {
+    var start = scanner.state;
+    expectIdentifier("url", ignoreCase: true);
+    var contents = _tryUrlContents(start);
+    if (contents != null) return new StringExpression(contents);
+
+    return new FunctionExpression(
+        new Interpolation(["url"], scanner.spanFrom(start)),
+        _argumentInvocation());
+  }
+
   /// Consumes tokens up to "{", "}", ";", or "!".
   ///
   /// This respects string and comment boundaries and supports interpolation.

lib/src/visitor/perform.dart

@@ -481,8 +481,9 @@ class PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
     return new CssMediaQuery(type, modifier: modifier, features: features);
   }
 
-  CssImport visitPlainImportRule(PlainImportRule node) =>
-      new CssImport(node.url, node.span);
+  void visitPlainImportRule(PlainImportRule node) {
+    _parent.addChild(new CssImport(_interpolationToValue(node.url), node.span));
+  }
 
   Value visitReturnRule(ReturnRule node) => node.expression.accept(this);
 

lib/src/visitor/serialize.dart

@@ -110,7 +110,7 @@ class _SerializeCssVisitor
   void visitImport(CssImport node) {
     _writeIndentation();
     _buffer.write("@import ");
-    _visitString(node.url.toString());
+    _buffer.write(node.url.value);
     _buffer.writeCharCode($semicolon);
   }