Dart2js testing: Add prefix files when running dart2js output on d8 or jsshell.

tools/testing/dart/runtime_configuration.dart

Index: tools/testing/dart/runtime_configuration.dart
diff --git a/tools/testing/dart/runtime_configuration.dart b/tools/testing/dart/runtime_configuration.dart
index fd30dd9931b165908781672498d332ab5ec5a407..6e36589030848a1e42cf07b2ccf7cf8bde760acf 100644
--- a/tools/testing/dart/runtime_configuration.dart
+++ b/tools/testing/dart/runtime_configuration.dart
@@ -16,6 +16,9 @@ import 'test_runner.dart' show
     Command,
     CommandBuilder;
 
+import 'utils.dart' show
+    Path;
+
 // TODO(ahe): I expect this class will become abstract very soon.
 class RuntimeConfiguration {
   // TODO(ahe): Remove this constructor and move the switch to
@@ -76,6 +79,8 @@ class RuntimeConfiguration {
     // TODO(ahe): Make this method abstract.
     throw "Unimplemented runtime '$runtimeType'";
   }
+
+  List<String> dart2jsPreambles(Path preambleDir) => [];
 }
 
 /// The 'none' runtime configuration.
@@ -124,6 +129,10 @@ class D8RuntimeConfiguration extends CommandLineJavaScriptRuntime {
         commandBuilder.getJSCommandlineCommand(
             moniker, suite.d8FileName, arguments, environmentOverrides)];
   }
+
+  List<String> dart2jsPreambles(Path preambleDir) {
+    return [preambleDir.append('d8.js').toNativePath()];
+  }
 }
 
 /// Firefox/SpiderMonkey-based development shell (jsshell).
@@ -142,6 +151,10 @@ class JsshellRuntimeConfiguration extends CommandLineJavaScriptRuntime {
         commandBuilder.getJSCommandlineCommand(
             moniker, suite.jsShellFileName, arguments, environmentOverrides)];
   }
+
+  List<String> dart2jsPreambles(Path preambleDir) {
+    return ['-f', preambleDir.append('jsshell.js').toNativePath(), '-f'];
+  }
 }
 
 /// Common runtime configuration for runtimes based on the Dart VM.