Change default build and test architecture to x64.

Precompilation isn't supported on IA32 so local testing misses it.

R=srdjan@google.com

Review URL: https://codereview.chromium.org/1408783005 .

tools/testing/dart/test_options.dart

@@ -107,7 +107,7 @@ class TestOptionsParser {
               ['-a', '--arch'],
               ['all', 'ia32', 'x64', 'arm', 'armv5te', 'arm64', 'mips',
                'simarm', 'simarmv5te', 'simarm64', 'simmips'],
-              'ia32'),
+              'x64'),
           new _TestOptionSpecification(
               'system',
               'The operating system to run tests on',

tools/utils.py

@@ -58,12 +58,12 @@ def GuessArchitecture():
     return 'arm64'
   elif os_id.startswith('mips'):
     return 'mips'
+  elif '64' in os_id:
+    return 'x64'
   elif (not os_id) or (not re.match('(x|i[3-6])86', os_id) is None):
     return 'ia32'
   elif os_id == 'i86pc':
     return 'ia32'
-  elif '64' in os_id:
-    return 'x64'
   else:
     guess_os = GuessOS()
     print "Warning: Guessing architecture %s based on os %s\n"\