cancel open subscription (#5824)

fixes https://github.com/flutter/flutter/issues/5788

.analysis_options

@@ -38,7 +38,7 @@ linter:
     # === error rules ===
     - avoid_empty_else
     # - comment_references # blocked on https://github.com/dart-lang/dartdoc/issues/1153
-    # - cancel_subscriptions # https://github.com/flutter/flutter/issues/5788
+    - cancel_subscriptions
     # - close_sinks # https://github.com/flutter/flutter/issues/5789
     - control_flow_in_finally
     - empty_statements

.analysis_options_flutter_analyze

@@ -39,7 +39,7 @@ linter:
     # === error rules ===
     - avoid_empty_else
     # - comment_references # blocked on https://github.com/dart-lang/dartdoc/issues/1153
-    # - cancel_subscriptions # https://github.com/flutter/flutter/issues/5788
+    - cancel_subscriptions
     # - close_sinks # https://github.com/flutter/flutter/issues/5789
     - control_flow_in_finally
     - empty_statements

packages/flutter_tools/lib/src/base/process.dart

@@ -78,6 +78,7 @@ Future<int> runCommandAndStreamOutput(List<String> cmd, {
   // Wait for stdout to be fully processed
   // because process.exitCode may complete first causing flaky tests.
   await subscription.asFuture();
+  subscription.cancel();
 
   return await process.exitCode;
 }