Convert most interfaces in dart:core to abstract classes.

lib/core/string_buffer.dart

Index: lib/core/string_buffer.dart
diff --git a/lib/core/string_buffer.dart b/lib/core/string_buffer.dart
index 9629ab7f8062a2982bc608de520589f4760e7070..8095f6a0c367a3b05ade9b494bd9ca36ec1fba57 100644
--- a/lib/core/string_buffer.dart
+++ b/lib/core/string_buffer.dart
@@ -7,11 +7,11 @@
  * efficiently. Only on a call to [toString] are the strings
  * concatenated to a single String.
  */
-interface StringBuffer default StringBufferImpl {
+abstract class StringBuffer {
   /**
    * Creates the string buffer with an initial content.
    */
-  StringBuffer([Object content]);
+  factory StringBuffer([Object content = ""]) => new StringBufferImpl(content);
 
   /**
    * Returns the length of the buffer.