Use abstract classes instead of interfaces in dart2js.

sdk/lib/_internal/compiler/implementation/scanner/token.dart

Index: sdk/lib/_internal/compiler/implementation/scanner/token.dart
diff --git a/sdk/lib/_internal/compiler/implementation/scanner/token.dart b/sdk/lib/_internal/compiler/implementation/scanner/token.dart
index 901202139bcdf4c3205b7c3b2e56229eef259021..a768b5038458be91960883793ce175f69e29d55d 100644
--- a/sdk/lib/_internal/compiler/implementation/scanner/token.dart
+++ b/sdk/lib/_internal/compiler/implementation/scanner/token.dart
@@ -177,8 +177,8 @@ class StringToken extends Token {
   String slowToString() => value.slowToString();
 }
 
-interface SourceString extends Iterable<int> default StringWrapper {
-  const SourceString(String string);
+abstract class SourceString extends Iterable<int> {
+  const factory SourceString(String string) = StringWrapper;
 
   void printOn(StringBuffer sb);