diff --git a/lib/css.dart b/lib/css.dart
index e7ef534..65f6511 100644
--- a/lib/css.dart
+++ b/lib/css.dart
@@ -35,7 +35,7 @@
     // Read the file.
     var filename = path.basename(inputPath);
     var contents = new File(inputPath).readAsStringSync();
-    var file = new SourceFile(contents, url: path.toUri(inputPath));
+    var file = new SourceFile.fromString(contents, url: path.toUri(inputPath));
 
     // Parse the CSS.
     StyleSheet tree = _time(
diff --git a/lib/parser.dart b/lib/parser.dart
index 32e27f5..db2068c 100644
--- a/lib/parser.dart
+++ b/lib/parser.dart
@@ -68,7 +68,7 @@
 
   _createMessages(errors: errors, options: options);
 
-  var file = new SourceFile(source);
+  var file = new SourceFile.fromString(source);
 
   var tree = new _Parser(file, source).parse();
 
@@ -99,7 +99,7 @@
 
   _createMessages(errors: errors, options: options);
 
-  var file = new SourceFile(source);
+  var file = new SourceFile.fromString(source);
   return new _Parser(file, source).parse();
 }
 
@@ -114,7 +114,7 @@
 
   _createMessages(errors: errors);
 
-  var file = new SourceFile(source);
+  var file = new SourceFile.fromString(source);
   return (new _Parser(file, source)..tokenizer.inSelector = true)
       .parseSelector();
 }
@@ -124,7 +124,7 @@
 
   _createMessages(errors: errors);
 
-  var file = new SourceFile(source);
+  var file = new SourceFile.fromString(source);
   return (new _Parser(file, source)
         // TODO(jmesserly): this fix should be applied to the parser. It's tricky
         // because by the time the flag is set one token has already been fetched.
diff --git a/pubspec.yaml b/pubspec.yaml
index 225b601..221d92b 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -9,7 +9,7 @@
   args: '>=0.9.0 <0.14.0'
   logging: '>=0.9.0 <0.12.0'
   path: '>=0.9.0 <2.0.0'
-  source_span: '>=1.0.0 <2.0.0'
+  source_span: '>=1.4.0 <2.0.0'
 dev_dependencies:
   browser: '>=0.9.0 <0.11.0'
   test: '>=0.12.0 <0.13.0'