Update SDK README to discuss about:tracing and observatory.

TBR=ianh@google.com

Review URL: https://codereview.chromium.org/1114553004

packages/flutter/CHANGELOG.md

+## 0.0.5+dart-summit-7
+
+  - Fix crash in sky_tool stop_tracing.
+
+## 0.0.5+dart-summit-6
+
+  - Fix missing include in sky_tool causing failure.
+
 ## 0.0.5+dart-summit-5
 
   - Added sky_tool start_tracing and stop_tracing.

packages/flutter/lib/sky_tool

@@ -13,6 +13,7 @@ import socket
 import subprocess
 import sys
 import urlparse
+import time
 
 # TODO(eseidel): This should be BIN_DIR.
 LIB_DIR = os.path.realpath(os.path.dirname(os.path.abspath(__file__)))