Oops, fix code coverage (#10672)

And add comments explaining why.

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

@@ -113,7 +113,8 @@ class _FlutterPlatform extends PlatformPlugin {
 
   @override
   StreamChannel<dynamic> loadChannel(String testPath, TestPlatform platform) {
-    if (enableObservatory || explicitDiagnosticPort != null) {
+    // Fail if there will be a port conflict.
+    if (explicitObservatoryPort != null || explicitDiagnosticPort != null) {
       if (_testCount > 0)
         throwToolExit('installHook() was called with an observatory port, a diagnostic port, both, or debugger mode enabled, but then more than one test suite was run.');
     }

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

@@ -32,6 +32,7 @@ Future<int> runTests(
     testArgs.addAll(<String>['--no-color', '-rexpanded']);
 
   if (enableObservatory) {
+    // (In particular, for collecting code coverage.)
     testArgs.add('--concurrency=1');
   }