Change the behavior of open on files to not truncate by default

runtime/bin/dartutils.cc

Index: runtime/bin/dartutils.cc
diff --git a/runtime/bin/dartutils.cc b/runtime/bin/dartutils.cc
index 9927c1a79e310a0749dd7e8a2f17bd3440af2a67..00f09564e4d2884eb65d38b742d61e69c8ee71ac 100644
--- a/runtime/bin/dartutils.cc
+++ b/runtime/bin/dartutils.cc
@@ -130,7 +130,7 @@ Dart_Handle DartUtils::CanonicalizeURL(CommandLineOptions* url_mapping,
 
 
 Dart_Handle DartUtils::ReadStringFromFile(const char* filename) {
-  File* file = File::Open(filename, false);
+  File* file = File::Open(filename, File::kRead);
   if (file == NULL) {
     const char* format = "Unable to open file: %s";
     intptr_t len = snprintf(NULL, 0, format, filename);