Don't include platform_sources in libvmservice_io
Don't include platform_sources in libvmservice_io

Fixes link error on Dartium Android:

/b/build/slave/dartium-android-release-be/build/src/third_party/android_tools/ndk/toolchains/arm-linux-androideabi-4.8/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.8/../../../../arm-linux-androideabi/bin/ld.gold: error: obj/dart/runtime/libdart_vm.a(obj/dart/runtime/platform/libdart_vm.assert.o): multiple definition of 'dart::DynamicAssertionHelper::Fail(char const*, ...)'
/b/build/slave/dartium-android-release-be/build/src/third_party/android_tools/ndk/toolchains/arm-linux-androideabi-4.8/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.8/../../../../arm-linux-androideabi/bin/ld.gold: obj/dart/runtime/libvmservice_io.a(obj/dart/runtime/platform/libvmservice_io.assert.o): previous definition here

Review URL: https://codereview.chromium.org//879083002

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@43217 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk