Observatory build should try backup method if the SDK doesn't work.

tools/utils.py

Index: tools/utils.py
diff --git a/tools/utils.py b/tools/utils.py
index ae04a53801880e84b69f6253c8567df1cf593ff9..9687fc2ed7783de770521e9b91c07ea5228c5798 100644
--- a/tools/utils.py
+++ b/tools/utils.py
@@ -565,6 +565,7 @@ def ExecuteCommand(cmd):
 
 
 def DartBinary():
+  # TODO(24311): Replace all uses of this with CheckedInSdk[Fix]Executable().
   tools_dir = os.path.dirname(os.path.realpath(__file__))
   dart_binary_prefix = os.path.join(tools_dir, 'testing', 'bin')
   if IsWindows():
@@ -592,6 +593,8 @@ def DartSdkBinary():
   return os.path.join(dart_binary_prefix, 'dart')
 
 
+# The checked-in SDKs are documented at
+#     https://github.com/dart-lang/sdk/wiki/The-checked-in-SDK-in--tools

[Ivan Posva, 2015/09/15 17:01:42]

ditto: --

[Bill Hesse, 2015/09/15 17:09:02]

This is the URL of the page.  I tracked it down, and found an extra space in the
page name.  Fixed.

 def CheckedInSdkPath():
   # We don't use the normal macos, linux, win32 directory names here, instead,
   # we use the names that the download_from_google_storage script uses.
@@ -609,6 +612,34 @@ def CheckedInSdkPath():
                       'dart-sdk')
 
 
+def CheckedInSdkExecutable():
+  return os.path.join(CheckedInSdkPath(),
+                      'bin',
+                      'dart.exe' if IsWindows() else 'dart')
+
+
+def CheckedInSdkFixExecutable():

[Ivan Posva, 2015/09/15 17:01:42]

The documentation talks about a CheckedInSdkFixedExecutable, but here we are
missing the Fixed and instead have a Fix, which reads really strangely. Please
update to the documented name.

[Bill Hesse, 2015/09/15 17:09:02]

I like Fix, because it is an active verb, and this function has an effect.  But
I will change to Fixed if others agree.

[Bill Hesse, 2015/09/21 10:05:06]

Changed to CheckedInSdkCheckExecutable, since it now only checks whether
CheckedInSdkExecutable works or not.

+  executable = CheckedInSdkExecutable()
+  canary_script = os.path.join(os.path.dirname(os.path.realpath(__file__)),
+                               'canary.dart')
+  try:
+    if 42 == subprocess.call([executable, canary_script]):
+      return True
+  except OSError as e:
+    pass
+  arch = GuessArchitecture()
+  system = GuessOS()
+  if system == 'linux' and (arch == 'mips' or arch == 'arm'):
+    try:
+      shutil.copy(os.path.join(CheckedInSdkPath(), 'bin', 'dart-%s' % arch),
+                  executable)

[Ivan Posva, 2015/09/15 17:01:42]

Here you are overwriting the ia32 executable which is really a bad thing to do
as you now have completely disabled running on a ia32 or x64 machine. Also it
breaks the "do-not-change" your source repository rule.

[Bill Hesse, 2015/09/15 17:09:02]

We are already changing the source repository by checking out this SDK into it
and unzipping it, at gclient runhooks.  If you want, I can put this change into
the gclient runhooks, and do it at that point.  There is no way to have a single
SDK checkout work for multiple architectures at once.  If this checkout is on an
ARM, MIPS, or other machine, the SDK on it should be the one with the proper
dart executable.  Even if I had separate SDKs up in the cloud, they would
download to the same place, so "dart" would be the one for the host.

[Bill Hesse, 2015/09/15 22:37:05]

We can avoid doing this, if you absolutely don't want to change the "dart" file,
by running pub always with an explicitly given dart executable, and the snapshot
from the SDK, rather than from the runnable pub command.  We would still want to
test that this command works, and fall back to the bootstrap executable if pub
doesn't run this way.

[Bill Hesse, 2015/09/15 22:37:05]

For testing purposes, I added a dart-ia32 binary to the repository, and
supported copying that to "dart" when building on ia32 or x64.  This could be
checked in, and then building would function on whatever platform the build was
currently running on.

[Ivan Posva, 2015/09/15 23:32:59]

Summarizing offline discussion here:
Specifying a dart executable is preferred over the copying the currently valid
executable over the dart binary. Thinking about it a bit further, we already
have a function which gives us the CheckedInSdkExecutable so the smarts of
figuring out the currect architecture can be added right there. No?

[Bill Hesse, 2015/09/21 10:05:06]

Done.

+      if 42 == subprocess.call([executable, canary_script]):
+        return True
+    except (OSError, IOError) as e:
+      pass
+  return False
+
+
 def CheckedInPubPath():
   executable_name = 'pub'
   if platform.system() == 'Windows':