first step for #168, adds dart:js impl to the build

tool/build_sdk.sh

 #!/bin/bash
 set -e
 # switch to the root directory of dev_compiler
 cd $( dirname "${BASH_SOURCE[0]}" )/..
 
 echo "*** Patching SDK"
 rm -r tool/generated_sdk || true
 dart -c tool/patch_sdk.dart tool/input_sdk tool/generated_sdk
 
 echo "*** Compiling SDK to JavaScript"
 if [[ -d lib/runtime/dart ]] ; then
   rm -r lib/runtime/dart
 fi
 
 # TODO(jmesserly): for now we're suppressing errors in SDK compilation
 dart -c bin/devc.dart --no-source-maps --sdk-check --force-compile -l warning \
-    --dart-sdk tool/generated_sdk -o lib/runtime/ dart:core \
+    --dart-sdk tool/generated_sdk -o lib/runtime/ dart:js \

[Jennifer Messerly, 2015/06/12 21:03:05]

JS includes core, so we still get it transitively.
We should support more than one source, though:
https://github.com/dart-lang/dev_compiler/issues/219

[Jacob, 2015/06/12 21:07:23]

make this a 
# comment in the shell script
having generate_sdk only build dart:js is a bit odd :)

[Jennifer Messerly, 2015/06/12 22:02:36]

haha. yes, good point.

     > tool/generated_sdk/sdk_errors.txt || true
 
 if [[ ! -f lib/runtime/dart/core.js ]] ; then
     echo 'core.js not found, assuming build failed.'
     echo './tool/build_sdk.sh can be run to reproduce this.'
     exit 1
 fi
 
 DIFF_ARGS="-u tool/sdk_expected_errors.txt tool/generated_sdk/sdk_errors.txt"
 
 if ! (diff $DIFF_ARGS > /dev/null) ; then
     diff $DIFF_ARGS |\
         sed -e "s/^\(+.*\)/\1/" |\
         sed -e "s/^\(-.*\)/\1/"
     echo
     echo 'SDK errors have changed.  To update expectations, run:'
     echo '$ cp tool/generated_sdk/sdk_errors.txt tool/sdk_expected_errors.txt'
     exit 1
 fi