Remove 'dartc' as an arch for build.py.

tools/utils.py

Index: tools/utils.py
===================================================================
--- tools/utils.py	(revision 892)
+++ tools/utils.py	(working copy)
@@ -136,12 +136,10 @@
 def GetBuildRoot(target_os, mode=None, arch=None):
   global BUILD_ROOT
   if mode:
-    path = os.path.join(BUILD_ROOT[target_os], GetBuildConf(mode, arch))
     # TODO(ngeoffray): Remove this test once the testing infrastructure does not
     # treat 'dartc' as an arch.
-    if not os.path.exists(path) and arch == 'dartc':
-      path = os.path.join(BUILD_ROOT[target_os], GetBuildConf(mode, ARCH_GUESS))
-    return path;
+    if arch == 'dartc': arch = ARCH_GUESS
+    return os.path.join(BUILD_ROOT[target_os], GetBuildConf(mode, arch))
   else:
     return BUILD_ROOT[target_os]