Better command line error messages

runtime/bin/main.cc

Index: runtime/bin/main.cc
diff --git a/runtime/bin/main.cc b/runtime/bin/main.cc
index 6c8ca4b2a23e5a5a3d1af7a9193603a0fe1906a5..d7bfb3b963c9bbf5137022514a9aaad2e26d9f59 100644
--- a/runtime/bin/main.cc
+++ b/runtime/bin/main.cc
@@ -207,7 +207,7 @@ static bool ProcessVerboseOption(const char* arg,
 static bool ProcessPackageRootOption(const char* arg,
                                      CommandLineOptions* vm_options) {
   ASSERT(arg != NULL);
-  if (*arg == '\0' || *arg == '-') {
+  if (*arg == '-') {
     return false;
   }
   commandline_package_root = arg;
@@ -216,9 +216,9 @@ static bool ProcessPackageRootOption(const char* arg,
 
 
 static bool ProcessPackagesOption(const char* arg,
-                                     CommandLineOptions* vm_options) {
+                                  CommandLineOptions* vm_options) {
   ASSERT(arg != NULL);
-  if ((*arg == '\0') || (*arg == '-')) {
+  if (*arg == '-') {
     return false;
   }
   commandline_packages_file = arg;
@@ -592,6 +592,16 @@ static int ParseArguments(int argc,
                   "file is invalid.\n");
     return -1;
   }
+  if ((commandline_package_root != NULL) &&
+      (strlen(commandline_package_root) == 0)) {
+    Log::PrintErr("Empty package root specified.\n");
+    return -1;
+  }
+  if ((commandline_packages_file != NULL) &&
+      (strlen(commandline_packages_file) == 0)) {
+    Log::PrintErr("Empty package file name specified.\n");
+    return -1;
+  }
   if (is_noopt && gen_snapshot_kind != kNone) {
     Log::PrintErr("Generating a snapshot with dart_noopt is invalid.\n");
     return -1;