Bump Dart SDK to 1.20.0-dev.10.2. (flutter#6272)

pq · web-flow · commit 6ec68c473450 · 2016-10-11T13:31:48.000-07:00
Picks up a patched `dartdoc` (effecting, notablly, `crossdart`…)
diff --git a/bin/internal/dart-sdk.version b/bin/internal/dart-sdk.version
@@ -1 +1 @@
-1.20.0-dev.10.1
+1.20.0-dev.10.2
