Clean up test_runner Command subclass hash/equality.

tools/testing/dart/compiler_configuration.dart

Index: tools/testing/dart/compiler_configuration.dart
diff --git a/tools/testing/dart/compiler_configuration.dart b/tools/testing/dart/compiler_configuration.dart
index 20ab1c77163774da2d5fa9021a06c82e0e88c58b..97822270347298282589cd67037d80b95a2c60e2 100644
--- a/tools/testing/dart/compiler_configuration.dart
+++ b/tools/testing/dart/compiler_configuration.dart
@@ -169,6 +169,8 @@ class NoneCompilerConfiguration extends CompilerConfiguration {
 /// dart2dart.
 class Dart2xCompilerConfiguration extends CompilerConfiguration {
   final String moniker;
+  static Map<String, List<Uri>> _bootstrapDependenciesCache =
+      new Map<String, List<Uri>>();
 
   Dart2xCompilerConfiguration(
       this.moniker,
@@ -214,10 +216,9 @@ class Dart2xCompilerConfiguration extends CompilerConfiguration {
 
   List<Uri> bootstrapDependencies(String buildDir) {
     if (!useSdk) return const <Uri>[];
-
-    Uri absoluteBuildDir = Uri.base.resolveUri(nativeDirectoryToUri(buildDir));
-    return [absoluteBuildDir.resolve(
-          'dart-sdk/bin/snapshots/dart2js.dart.snapshot')];
+    return _bootstrapDependenciesCache.putIfAbsent(buildDir, () =>
+      [Uri.base.resolveUri(nativeDirectoryToUri(buildDir))
+               .resolve('dart-sdk/bin/snapshots/dart2js.dart.snapshot')]);
   }
 }