Cache output of dart2js compilations that went wrong on disk.

tools/testing/dart/test_suite.dart

Index: tools/testing/dart/test_suite.dart
===================================================================
--- tools/testing/dart/test_suite.dart	(revision 36157)
+++ tools/testing/dart/test_suite.dart	(working copy)
@@ -1917,6 +1917,23 @@
         .pipe(new File(dest.toNativePath()).openWrite());
   }
 
+  static Future isContentEqual(Path source, Path dest) {
+    return new File(source.toNativePath()).readAsString()
+      .then((srcString) {
+        return new File(dest.toNativePath()).readAsString()
+          .then((destString) => srcString == destString);
+      })
+      .catchError((error) {
+          return false;
+      });
+  }
+
+  static Future copyIfDifferent(Path source, Path dest) {
+    return isContentEqual(source, dest).then((equal) {
+      return equal ? null : TestUtils.copyFile(source, dest);
+    });
+  }
+
   static Future copyDirectory(String source, String dest) {
     source = new Path(source).toNativePath();
     dest = new Path(dest).toNativePath();
@@ -2147,6 +2164,11 @@
     }
     return path;
   }
+
+  static String cachedOutputFile(String outputFile) {
+    return "$outputFile.cached_output";
+  }
+
 }
 
 class SummaryReport {