Make 'tools/build.py -h' warn the user to run it from the repo root.

tools/build.py

Index: tools/build.py
diff --git a/tools/build.py b/tools/build.py
index 4ca626230520a00ca8122fdd4e0705ea7fde9ace..805bca5711379e5e562781458b4440388e895605 100755
--- a/tools/build.py
+++ b/tools/build.py
@@ -511,6 +511,19 @@ def BuildCrossSdk(options, target_os, mode, arch):
 
 
 def Main():
+  # This script assumes it is running from the repo root and can
+  # silently fail when that's not true. A more user friendly option
+  # would be to make it run from anywhere. For now we just fail early
+  # and loudly.
+  if os.path.abspath(utils.DART_DIR) != os.path.abspath(os.getcwd()):

[Ivan Posva, 2014/09/17 03:48:55]

This is not entirely correct. Sometimes you need to run the build.py script from
within a subdirectory of the dart repo.

+    print 'This script must be run from the repo root!'
+    print 'Please cd into'
+    print ''
+    print '  %s' % utils.DART_DIR
+    print ''
+    print 'before running.'
+    return 1
+
   utils.ConfigureJava()
   # Parse the options.
   parser = BuildOptions()