Reapply conversion of most core interfaces to abstract classes.

lib/coreimpl/string_buffer.dart

Index: lib/coreimpl/string_buffer.dart
diff --git a/lib/coreimpl/string_buffer.dart b/lib/coreimpl/string_buffer.dart
index e59643e6009ae6af0a8cc4f599cfe89496febff9..ceb59350d3a43136d3f5eb0ccb9b0f84a25e7b22 100644
--- a/lib/coreimpl/string_buffer.dart
+++ b/lib/coreimpl/string_buffer.dart
@@ -11,7 +11,7 @@ class StringBufferImpl implements StringBuffer {
   /**
    * Creates the string buffer with an initial content.
    */
-  StringBufferImpl([Object content = ""]) {
+  StringBufferImpl(Object content) {
     clear();
     add(content);
   }