Add StreamBuffer to dart:io.

tests/standalone/io/stream_buffer_test.dart

+#import("dart:io");
+#import("dart:isolate");
+#import("dart:math");
+
+#source("stream_buffer.dart");
+
+void main() {
+  var buffer = new StreamBuffer();
+  var out = buffer.toBuffer;
+  var een = buffer.fromBuffer;
+  een.onData = () {
+    var first = new List<int>(6);
+    een.readInto(first, 1, 4);
+    print(first);
+    een.onData = () {
+      var data = een.read(2);
+      print(data);
+    };
+  };
+  een.onClosed = () {
+    print('buffer closed');
+  };
+
+  out.write([23, 17, 19]);
+  out.write([27, 35]);
+  out.write([56, 98]);
+  out.close();
+}