Add flutter_engine runtime to the test script so we can run standard dart tests from the test suite…

tools/testing/dart/test_suite.dart

Index: tools/testing/dart/test_suite.dart
diff --git a/tools/testing/dart/test_suite.dart b/tools/testing/dart/test_suite.dart
index 47b8a8791b67dcf9e333f435677375bbb708a3c2..c256d257666230e8cc37f213717c45ce18431e3b 100644
--- a/tools/testing/dart/test_suite.dart
+++ b/tools/testing/dart/test_suite.dart
@@ -208,6 +208,19 @@ abstract class TestSuite {
     return dartExecutable;
   }
 
+  /// Returns the name of the Sky shell executable.

[Bill Hesse, 2017/01/05 17:48:33]

Should we use the name "flutter engine" everywhere, instead of both "flutter
engine" and "Sky shell"?

[siva, 2017/01/05 20:38:45]

Done.

+  String get skyBinaryFileName {
+    // Controlled by user with the option "--engine".
+    String skyExecutable = configuration['engine'];

[Bill Hesse, 2017/01/05 17:48:33]

If the default for this value in test_options was null, then you could just
write
configuration['engine'] ?? 'sky_shell'
Or the default should just be 'sky_shell' in the test_options option
declaration.

This code is in contradiction to the code in test_options which disallows this
case.

[siva, 2017/01/05 20:38:45]

Removed this default setting, the option is required for running flutter.

+
+    if (skyExecutable == '') {
+      skyExecutable = 'sky_shell';
+    }
+
+    TestUtils.ensureExists(skyExecutable, configuration);
+    return skyExecutable;
+  }
+
   String get dartVmNooptBinaryFileName {
     // Controlled by user with the option "--dart".
     String dartExecutable = configuration['dart'];