frog build relies on release build being present

[iposva-google]

The build scripts rely on unbuilt stuff. Just building --mode=debug will fail with this message:

PhaseScriptExecution "Action "generate_frogsh"" /Sources/dart-all/dart/xcodebuild/dart-frog.build/Debug_ia32/frogsh.build/Script-095A7D91DE56B603C848FB82.sh
    cd /Sources/dart-all/dart/frog
    /bin/sh -c /Sources/dart-all/dart/xcodebuild/dart-frog.build/Debug_ia32/frogsh.build/Script-095A7D91DE56B603C848FB82.sh

note: Generating frogsh file
Dart VM not built. Please run the following command:
 ../tools/build.py -m release
Command /bin/sh failed with exit code 1
Command /bin/sh failed with exit code 1

This is broken on many levels, as you rebuild debug in will keep using an out-of-date Dart VM binary. This build needs to honor the current(!) build mode.

[iposva-google]

cc @dgrove.

[dgrove]

Added Duplicate label.
Marked as being merged into #669.