Switch to concurrency of two, to see if that is a good balance. (#13003)

packages/flutter_tools/lib/src/test/runner.dart

@@ -38,6 +38,16 @@ Future<int> runTests(
     testArgs.addAll(<String>['-r', 'json']);
   }
 
+  if (enableObservatory) {
+    // (In particular, for collecting code coverage.)
+
+    // Turn on concurrency, but just barely.  If we just go with full concurrency, then
+    // individual tests timeout.  If we turn it off (=1), then the overall tests timeout.
+    // This is a lit fuse... Eventually it won't matter what number we put here.
+    // TODO(gspencer): Fix this: https://github.com/flutter/flutter/issues/10694
+    testArgs.add('--concurrency=2');
+  }
+
   for (String name in names) {
     testArgs..add('--name')..add(name);
   }